2. Language Definition 2.1 Objects 2.1.1 Atoms and Sequences All data objects in Euphoria are either atoms or sequences. An atom is a single numeric value. A sequence is a collection of numeric values. The objects contained in a sequence can be an arbitrary mix of atoms or sequences. A sequence is represented by a list of objects in brace brackets, separated by commas. Atoms can have any integer or double-precision floating point value. They can range from approximately -1e300 (minus one times 10 to the power 300) to +1e300 with 15 decimal digits of accuracy. Here are some Euphoria objects: ``` -- examples of atoms: 0 1000 98.6 -1e6 -- examples of sequences: {2, 3, 5, 7, 11, 13, 17, 19} {1, 2, {3, 3, 3}, 4, {5, {6}}} {{"jon", "smith"}, 52389, 97.25} {} -- the 0-element sequence ``` Numbers can also be entered in hexadecimal. For example: ``` #FE -- 254 #A000 -- 40960 #FFFF00008 -- 68718428168 -#10 -- -16 ``` Only the capital letters A, B, C, D, E, F are allowed in hex numbers. Hex numbers are always positive, unless you add a minus sign in front of the # character. So for instance #FFFFFFFF is a huge positive number (4294967295), *not* -1, as some machine-language programmers might expect. Sequences can be nested to any depth, i.e. you can have sequences within sequences within sequences and so on to any depth (until you run out of memory). Brace brackets are used to construct sequences out of a list of expressions. These expressions can be constant or evaluated at run-time. e.g. ``` {x+6, 9, y*w+2, sin(0.5)} ``` The "Hierarchical Objects" part of the Euphoria acronym comes from the hierarchical nature of nested sequences. This should not be confused with the class hierarchies of certain object-oriented languages. Why do we call them atoms? Why not just "numbers"? Well, an atom is just a number, but we wanted to have a distinctive term that emphasizes that they are indivisible. Of course in the world of physics, atoms were split into smaller parts many years ago, but in Euphoria you can't split them. They are the basic building blocks of all the data that a Euphoria program can manipulate. With this analogy, sequences might be thought of as "molecules", made from atoms and other molecules. A better analogy would be that sequences are like directories, and atoms are like files. Just as a directory on your computer can contain both files and other directories, a sequence can contain both atoms and other sequences (and those sequences can contain atoms and sequences and so on). As you will soon discover, sequences make Euphoria very simple and very powerful. Understanding atoms and sequences is the key to understanding Euphoria. Performance Note: Does this mean that all atoms are stored in memory as 8-byte floating-point numbers? No. The Euphoria interpreter usually stores integer-valued atoms as machine integers (4 bytes) to save space and improve execution speed. When fractional results occur or numbers get too big, conversion to floating-point happens automatically. 2.1.2 Character Strings and Individual Characters A character string is just a sequence of characters. It may be entered using quotes e.g. ``` "ABCDEFG" ``` Character strings may be manipulated and operated upon just like any other sequences. For example the above string is entirely equivalent to the sequence: ``` {65, 66, 67, 68, 69, 70, 71} ``` which contains the corresponding ASCII codes. The Euphoria compiler will immediately convert "ABCDEFG" to the above sequence of numbers. In a sense, there are no "strings" in Euphoria, only sequences of numbers. A quoted string is really just a convenient notation that saves you from having to type in all the ASCII codes. It follows that "" is equivalent to {}. Both represent the sequence of length-0, also known as the empty sequence. As a matter of programming style, it is natural to use "" to suggest a length-0 sequence of characters, and {} to suggest some other kind of sequence. An individual character is an atom. It must be entered using single quotes. There is a difference between an individual character (which is an atom), and a character string of length-1 (which is a sequence). e.g. ``` 'B' -- equivalent to the atom 66 - the ASCII code for B "B" -- equivalent to the sequence {66} ``` Again, 'B' is just a notation that is equivalent to typing 66. There aren't really any "characters" in Euphoria, just numbers (atoms). Keep in mind that an atom is not equivalent to a one-element sequence containing the same value, although there are a few built-in routines that choose to treat them similarly. Special characters may be entered using a back-slash: ``` \n newline \r carriage return \t tab \\ backslash \" double quote \' single quote ``` For example, "Hello, World!\n", or '\\'. The Euphoria editor displays character strings in green. 2.1.3 Comments Comments are started by two dashes and extend to the end of the current line. e.g. ``` -- this is a comment ``` Comments are ignored by the compiler and have no effect on execution speed. The editor displays comments in red. On the first line (only) of your program, you can use a special comment beginning with #!, e.g. ``` #!/home/rob/euphoria/bin/exu ``` This informs the Linux shell that your file should be executed by the Euphoria interpreter, and gives the full path to the interpreter. If you make your file executable, you can run it, just by typing its name, and without the need to type "exu". On DOS and Windows this line is just treated as a comment (though Apache Web server on Windows does recognize it.). If your file is a shrouded .il file, use backendu instead of exu. 2.2 Expressions Like other programming languages, Euphoria lets you calculate results by forming expressions. However, in Euphoria you can perform calculations on entire sequences of data with one expression, where in most other languages you would have to construct a loop. In Euphoria you can handle a sequence much as you would a single number. It can be copied, passed to a subroutine, or calculated upon as a unit. For example, ``` {1,2,3} + 5 ``` is an expression that adds the sequence {1,2,3} and the atom 5 to get the resulting sequence {6,7,8}. We will see more examples later. 2.2.1 Relational Operators The relational operators <   >   <=   >=   =   !=   each produce a 1 (true) or a 0 (false) result. ``` 8.8 < 8.7 -- 8.8 less than 8.7 (false) -4.4 > -4.3 -- -4.4 greater than -4.3 (false) 8 <= 7 -- 8 less than or equal to 7 (false) 4 >= 4 -- 4 greater than or equal to 4 (true) 1 = 10 -- 1 equal to 10 (false) 8.7 != 8.8 -- 8.7 not equal to 8.8 (true) ``` As we will soon see you can also apply these operators to sequences. 2.2.2 Logical Operators The logical operators and, or, xor, and not are used to determine the "truth" of an expression. e.g. ``` 1 and 1 -- 1 (true) 1 and 0 -- 0 (false) 0 and 1 -- 0 (false) 0 and 0 -- 0 (false) 1 or 1 -- 1 (true) 1 or 0 -- 1 (true) 0 or 1 -- 1 (true) 0 or 0 -- 0 (false) 1 xor 1 -- 0 (false) 1 xor 0 -- 1 (true) 0 xor 1 -- 1 (true) 0 xor 0 -- 0 (false) not 1 -- 0 (false) not 0 -- 1 (true) ``` You can also apply these operators to numbers other than 1 or 0. The rule is: zero means false and non-zero means true. So for instance: ``` 5 and -4 -- 1 (true) not 6 -- 0 (false) ``` These operators can also be applied to sequences. See below. In some cases short-circuit evaluation will be used for expressions containing and or or. 2.2.3 Arithmetic Operators The usual arithmetic operators are available: add, subtract, multiply, divide, unary minus, unary plus. ``` 3.5 + 3 -- 6.5 3 - 5 -- -2 6 * 2 -- 12 7 / 2 -- 3.5 -8.1 -- -8.1 +8 -- +8 ``` Computing a result that is too big (i.e. outside of -1e300 to +1e300) will result in one of the special atoms +infinity or -infinity. These appear as inf or -inf when you print them out. It is also possible to generate nan or -nan. "nan" means "not a number", i.e. an undefined value (such as inf divided by inf). These values are defined in the IEEE floating-point standard. If you see one of these special values in your output, it usually indicates an error in your program logic, although generating inf as an intermediate result may be acceptable in some cases. For instance, 1/inf is 0, which may be the "right" answer for your algorithm. Division by zero, as well as bad arguments to math library routines, e.g. square root of a negative number, log of a non-positive number etc. cause an immediate error message and your program is aborted. The only reason that you might use unary plus is to emphasize to the reader of your program that a number is positive. The interpreter does not actually calculate anything for this. 2.2.4 Operations on Sequences All of the relational, logical and arithmetic operators described above, as well as the math routines described in Part II - Library Routines, can be applied to sequences as well as to single numbers (atoms). When applied to a sequence, a unary (one operand) operator is actually applied to each element in the sequence to yield a sequence of results of the same length. If one of these elements is itself a sequence then the same rule is applied again recursively. e.g. ``` x = -{1, 2, 3, {4, 5}} -- x is {-1, -2, -3, {-4, -5}} ``` If a binary (two-operand) operator has operands which are both sequences then the two sequences must be of the same length. The binary operation is then applied to corresponding elements taken from the two sequences to get a sequence of results. e.g. ``` x = {5, 6, 7, 8} + {10, 10, 20, 100} -- x is {15, 16, 27, 108} ``` If a binary operator has one operand which is a sequence while the other is a single number (atom) then the single number is effectively repeated to form a sequence of equal length to the sequence operand. The rules for operating on two sequences then apply. Some examples: ``` y = {4, 5, 6} w = 5 * y -- w is {20, 25, 30} x = {1, 2, 3} z = x + y -- z is {5, 7, 9} z = x < y -- z is {1, 1, 1} w = {{1, 2}, {3, 4}, {5}} w = w * y -- w is {{4, 8}, {15, 20}, {30}} w = {1, 0, 0, 1} and {1, 1, 1, 0} -- {1, 0, 0, 0} w = not {1, 5, -2, 0, 0} -- w is {0, 0, 0, 1, 1} w = {1, 2, 3} = {1, 2, 4} -- w is {1, 1, 0} -- note that the first '=' is assignment, and the -- second '=' is a relational operator that tests -- equality ``` Note: When you wish to compare two strings (or other sequences), you should not (as in some other languages) use the '=' operator: ``` if "APPLE" = "ORANGE" then -- ERROR! ``` '=' is treated as an operator, just like '+', '*' etc., so it is applied to corresponding sequence elements, and the sequences must be the same length. When they are equal length, the result is a sequence of 1's an 0's. When they are not equal length, the result is an error. Either way you'll get an error, since an if-condition must be an atom, not a sequence. Instead you should use the equal() built-in routine: ``` if equal("APPLE", "ORANGE") then -- CORRECT ``` In general, you can do relational comparisons using the compare() built-in routine: ``` if compare("APPLE", "ORANGE") = 0 then -- CORRECT ``` You can use compare() for other comparisons as well: ``` if compare("APPLE", "ORANGE") < 0 then -- CORRECT -- enter here if "APPLE" is less than "ORANGE" (TRUE) ``` 2.2.5 Subscripting of Sequences A single element of a sequence may be selected by giving the element number in square brackets. Element numbers start at 1. Non-integer subscripts are rounded down to an integer. For example, if x contains {5, 7.2, 9, 0.5, 13} then x[2] is 7.2. Suppose we assign something different to x[2]: ``` x[2] = {11,22,33} ``` Then x becomes: {5, {11,22,33}, 9, 0.5, 13}. Now if we ask for x[2] we get {11,22,33} and if we ask for x[2][3] we get the atom 33. If you try to subscript with a number that is outside of the range 1 to the number of elements, you will get a subscript error. For example x[0], x[-99] or x[6] will cause errors. So will x[1][3] since x[1] is not a sequence. There is no limit to the number of subscripts that may follow a variable, but the variable must contain sequences that are nested deeply enough. The two dimensional array, common in other languages, can be easily represented with a sequence of sequences: ``` x = { {5, 6, 7, 8, 9}, -- x[1] {1, 2, 3, 4, 5}, -- x[2] {0, 1, 0, 1, 0} -- x[3] } ``` where we have written the numbers in a way that makes the structure clearer. An expression of the form x[i][j] can be used to access any element. The two dimensions are not symmetric however, since an entire "row" can be selected with x[i], but there is no simple expression to select an entire column. Other logical structures, such as n-dimensional arrays, arrays of strings, structures, arrays of structures etc. can also be handled easily and flexibly: ``` 3-D array: y = { {{1,1}, {3,3}, {5,5}}, {{0,0}, {0,1}, {9,1}}, {{-1,9},{1,1}, {2,2}} } ``` y[2][3][1] is 9 Array of strings: ``` s = {"Hello", "World", "Euphoria", "", "Last One"} ``` s[3] is "Euphoria" s[3][1] is 'E' A Structure: ``` employee = { {"John","Smith"}, 45000, 27, 185.5 } ``` To access "fields" or elements within a structure it is good programming style to make up a set of constants that name the various fields. This will make your program easier to read. For the example above you might have: ``` constant NAME = 1 constant FIRST_NAME = 1, LAST_NAME = 2 constant SALARY = 2 constant AGE = 3 constant WEIGHT = 4 ``` You could then access the person's name with employee[NAME], or if you wanted the last name you could say employee[NAME][LAST_NAME]. Array of structures: ``` employees = { {{"John","Smith"}, 45000, 27, 185.5}, -- a[1] {{"Bill","Jones"}, 57000, 48, 177.2}, -- a[2] -- .... etc. } ``` employees[2][SALARY] would be 57000. The length() built-in function will tell you the length of a sequence. So the last element of a sequence s, is: ``` s[length(s)] ``` A short-hand for this is: ``` s[\$] ``` Similarly, ``` s[length(s)-1] ``` can be simplified to: ``` s[\$-1] ``` The \$ symbol equals the length of the sequence. \$ may only appear between square braces. Where there's nesting, e.g.: ``` s[\$ - t[\$-1] + 1] ``` The first \$ above refers to the length of s, while the second \$ refers to the length of t (as you'd probably expect). An example where \$ can save a lot of typing, make your code clearer, and probably even faster is: ``` longname[\$][\$] -- last element of the last element ``` Compare that with the equivalent: ``` longname[length(longname)][length(longname[length(longname)])] ``` Subscripting and function side-effects: In an assignment statement, with left-hand-side subscripts: ``` lhs_var[lhs_expr1][lhs_expr2]... = rhs_expr ``` The expressions are evaluated, and any subscripting is performed, from left to right. It is possible to have function calls in the right-hand-side expression, or in any of the left-hand-side expressions. If a function call has the side-effect of modifying the lhs_var, it is not defined whether those changes will appear in the final value of the lhs_var, once the assignment has been completed. To be sure about what is going to happen, perform the function call in a separate statement, i.e. do not try to modify the lhs_var in two different ways in the same statement. Where there are no left-hand-side subscripts, you can always assume that the final value of the lhs_var will be the value of rhs_expr, regardless of any side-effects that may have changed lhs_var. Euphoria data structures are almost infinitely flexible. Arrays in other languages are constrained to have a fixed number of elements, and those elements must all be of the same type. Euphoria eliminates both of those restrictions. You can easily add a new structure to the employee sequence above, or store an unusually long name in the NAME field and Euphoria will take care of it for you. If you wish, you can store a variety of different employee "structures", with different sizes, all in one sequence. Not only can a Euphoria program easily represent all conventional data structures but you can create very useful, flexible structures that would be extremely hard to declare in a conventional language. See 2.3 Euphoria versus Conventional Languages. Note that expressions in general may not be subscripted, just variables. For example: {5+2,6-1,7*8,8+1}[3] is not supported, nor is something like: date()[MONTH]. You have to assign the sequence returned by date() to a variable, then subscript the variable to get the month. 2.2.6 Slicing of Sequences A sequence of consecutive elements may be selected by giving the starting and ending element numbers. For example if x is {1, 1, 2, 2, 2, 1, 1, 1} then x[3..5] is the sequence {2, 2, 2}. x[3..3] is the sequence {2}. x[3..2] is also allowed. It evaluates to the length-0 sequence {}. If y has the value: {"fred", "george", "mary"} then y[1..2] is {"fred", "george"}. We can also use slices for overwriting portions of variables. After x[3..5] = {9, 9, 9} x would be {1, 1, 9, 9, 9, 1, 1, 1}. We could also have said x[3..5] = 9 with the same effect. Suppose y is {0, "Euphoria", 1, 1}. Then y[2][1..4] is "Euph". If we say y[2][1..4]="ABCD" then y will become {0, "ABCDoria", 1, 1}. In general, a variable name can be followed by 0 or more subscripts, followed in turn by 0 or 1 slices. Only variables may be subscripted or sliced, not expressions. We need to be a bit more precise in defining the rules for empty slices. Consider a slice s[i..j] where s is of length n. A slice from i to j, where j = i-1 and i >= 1 produces the empty sequence, even if i = n+1. Thus 1..0 and n+1..n and everything in between are legal (empty) slices. Empty slices are quite useful in many algorithms. A slice from i to j where j < i - 1 is illegal , i.e. "reverse" slices such as s[5..3] are not allowed. We can also use the \$ shorthand with slices, e.g. ``` s[2..\$] s[5..\$-2] s[\$-5..\$] s[\$][1..floor(\$/2)] -- first half of the last element of s ``` 2.2.7 Concatenation of Sequences and Atoms - The '&' Operator Any two objects may be concatenated using the & operator. The result is a sequence with a length equal to the sum of the lengths of the concatenated objects (where atoms are considered here to have length 1). e.g. ``` {1, 2, 3} & 4 -- {1, 2, 3, 4} 4 & 5 -- {4, 5} {{1, 1}, 2, 3} & {4, 5} -- {{1, 1}, 2, 3, 4, 5} x = {} y = {1, 2} y = y & x -- y is still {1, 2} ``` You can delete element i of any sequence s by concatenating the parts of the sequence before and after i: ``` s = s[1..i-1] & s[i+1..length(s)] ``` This works even when i is 1 or length(s), since s[1..0] is a legal empty slice, and so is s[length(s)+1..length(s)]. 2.2.8 Sequence-Formation Finally, sequence-formation, using braces and commas: ``` {a, b, c, ... } ``` is also an operator. It takes n operands, where n is 0 or more, and makes an n-element sequence from their values. e.g. ``` x = {apple, orange*2, {1,2,3}, 99/4+foobar} ``` The sequence-formation operator is listed at the bottom of the precedence chart. 2.2.9 Other Operations on Sequences Some other important operations that you can perform on sequences have English names, rather than special characters. These operations are built-in to ex.exe/exw.exe/exu , so they'll always be there, and so they'll be fast. They are described in detail in Part II - Library Routines, but are important enough to Euphoria programming that we should mention them here before proceeding. You call these operations as if they were subroutines, although they are actually implemented much more efficiently than that. length(s) length() tells you the length of a sequence s. This is the number of elements in s. Some of these elements may be sequences that contain elements of their own, but length just gives you the "top-level" count. You'll get an error if you ask for the length of an atom. e.g. ``` length({5,6,7}) -- 3 length({1, {5,5,5}, 2, 3}) -- 4 (not 6!) length({}) -- 0 length(5) -- error! ``` repeat(item, count) repeat() makes a sequence that consists of an item repeated count times. e.g. ``` repeat(0, 100) -- {0,0,0,...,0} i.e. 100 zeros repeat("Hello", 3) -- {"Hello", "Hello", "Hello"} repeat(99,0) -- {} ``` The item to be repeated can be any atom or sequence. append(s, item) / prepend(s, item) append() creates a new sequence by adding an item to the end of a sequence s. prepend() creates a new sequence by adding an element to the beginning of a sequence s. e.g. ``` append({1,2,3}, 4) -- {1,2,3,4} prepend({1,2,3}, 4) -- {4,1,2,3} append({1,2,3}, {5,5,5}) -- {1,2,3,{5,5,5}} prepend({}, 9) -- {9} append({}, 9) -- {9} ``` The length of the new sequence is always 1 greater than the length of the original sequence. The item to be added to the sequence can be any atom or sequence. These two built-in functions, append() and prepend(), have some similarities to the concatenate operator, &, but there are clear differences. e.g. ``` -- appending a sequence is different append({1,2,3}, {5,5,5}) -- {1,2,3,{5,5,5}} {1,2,3} & {5,5,5} -- {1,2,3,5,5,5} -- appending an atom is the same append({1,2,3}, 5) -- {1,2,3,5} {1,2,3} & 5 -- {1,2,3,5} ``` 2.2.10 Precedence Chart The precedence of operators in expressions is as follows: ``` highest precedence: function/type calls unary- unary+ not * / + - & < > <= >= = != and or xor lowest precedence: { , , , } ``` Thus 2+6*3 means 2+(6*3) rather than (2+6)*3. Operators on the same line above have equal precedence and are evaluated left to right. You can force any order of operations by placing round brackets ( ) around an expression. The equals symbol '=' used in an assignment statement is not an operator, it's just part of the syntax of the language. 2.3 Euphoria versus Conventional Languages By basing Euphoria on this one, simple, general, recursive data structure, a tremendous amount of the complexity normally found in programming languages has been avoided. The arrays, structures, unions, arrays of records, multidimensional arrays, etc. of other languages can all be easily represented in Euphoria with sequences. So can higher-level structures such as lists, stacks, queues, trees etc. Furthermore, in Euphoria you can have sequences of mixed type; you can assign any object to an element of a sequence; and sequences easily grow or shrink in length without your having to worry about storage allocation issues. The exact layout of a data structure does not have to be declared in advance, and can change dynamically as required. It is easy to write generic code, where, for instance, you push or pop a mix of various kinds of data objects using a single stack. Making a flexible list that contains a variety of different kinds of data objects is trivial in Euphoria, but requires dozens of lines of ugly code in other languages. Data structure manipulations are very efficient since the Euphoria interpreter will point to large data objects rather than copy them. Programming in Euphoria is based entirely on creating and manipulating flexible, dynamic sequences of data. Sequences are it - there are no other data structures to learn. You operate in a simple, safe, elastic world of values, that is far removed from the rigid, tedious, dangerous world of bits, bytes, pointers and machine crashes. Unlike other languages such as LISP and Smalltalk, Euphoria's "garbage collection" of unused storage is a continuous process that never causes random delays in execution of a program, and does not pre-allocate huge regions of memory. The language definitions of conventional languages such as C, C++, Ada, etc. are very complex. Most programmers become fluent in only a subset of the language. The ANSI standards for these languages read like complex legal documents. You are forced to write different code for different data types simply to copy the data, ask for its current length, concatenate it, compare it etc. The manuals for those languages are packed with routines such as "strcpy", "strncpy", "memcpy", "strcat", "strlen", "strcmp", "memcmp", etc. that each only work on one of the many types of data. Much of the complexity surrounds issues of data type. How do you define new types? Which types of data can be mixed? How do you convert one type into another in a way that will keep the compiler happy? When you need to do something requiring flexibility at run-time, you frequently find yourself trying to fake out the compiler. In these languages the numeric value 4 (for example) can have a different meaning depending on whether it is an int, a char, a short, a double, an int * etc. In Euphoria, 4 is the atom 4, period. Euphoria has something called types as we shall see later, but it is a much simpler concept. Issues of dynamic storage allocation and deallocation consume a great deal of programmer coding time and debugging time in these other languages, and make the resulting programs much harder to understand. Programs that must run continuously often exhibit storage "leaks", since it takes a great deal of discipline to safely and properly free all blocks of storage once they are no longer needed. Pointer variables are extensively used. The pointer has been called the "go to" of data structures. It forces programmers to think of data as being bound to a fixed memory location where it can be manipulated in all sorts of low-level, non-portable, tricky ways. A picture of the actual hardware that your program will run on is never far from your mind. Euphoria does not have pointers and does not need them. 2.4 Declarations 2.4.1 Identifiers Identifiers, which consist of variable names and other user-defined symbols, may be of any length. Upper and lower case are distinct. Identifiers must start with a letter and then be followed by letters, digits or underscores. The following reserved words have special meaning in Euphoria and may not be used as identifiers: ``` and end include to by exit not type constant for or while do function procedure with else global return without elsif if then xor ``` The Euphoria editor displays these words in blue. Identifiers can be used in naming the following: procedures functions types variables constants procedures These perform some computation and may have a list of parameters, e.g. ``` procedure empty() end procedure procedure plot(integer x, integer y) position(x, y) puts(1, '*') end procedure ``` There are a fixed number of named parameters, but this is not restrictive since any parameter could be a variable-length sequence of arbitrary objects. In many languages variable-length parameter lists are impossible. In C, you must set up strange mechanisms that are complex enough that the average programmer cannot do it without consulting a manual or a local guru. A copy of the value of each argument is passed in. The formal parameter variables may be modified inside the procedure but this does not affect the value of the arguments. Performance Note: The interpreter does not actually copy sequences or floating-point numbers unless it becomes necessary. For example, ``` y = {1,2,3,4,5,6,7,8.5,"ABC"} x = y ``` The statement x = y does not actually cause a new copy of y to be created. Both x and y will simply "point" to the same sequence. If we later perform x[3] = 9, then a separate sequence will be created for x in memory (although there will still be just one shared copy of 8.5 and "ABC"). The same thing applies to "copies" of arguments passed in to subroutines. functions These are just like procedures, but they return a value, and can be used in an expression, e.g. ``` function max(atom a, atom b) if a >= b then return a else return b end if end function ``` Any Euphoria object can be returned. You can, in effect, have multiple return values, by returning a sequence of objects. e.g. ``` return {x_pos, y_pos} ``` We will use the general term "subroutine", or simply "routine" when a remark is applicable to both procedures and functions. types These are special functions that may be used in declaring the allowed values for a variable. A type must have exactly one parameter and should return an atom that is either true (non-zero) or false (zero). Types can also be called just like other functions. See 2.4.3 Specifying the Type of a Variable. variables These may be assigned values during execution e.g. ``` -- x may only be assigned integer values integer x x = 25 -- a, b and c may be assigned *any* value object a, b, c a = {} b = a c = 0 ``` When you declare a variable you name the variable (which protects you against making spelling mistakes later on) and you specify the values that may legally be assigned to the variable during execution of your program. constants These are variables that are assigned an initial value that can never change e.g. ``` constant MAX = 100 constant Upper = MAX - 10, Lower = 5 constant name_list = {"Fred", "George", "Larry"} ``` The result of any expression can be assigned to a constant, even one involving calls to previously defined functions, but once the assignment is made, the value of the constant variable is "locked in". Constants may not be declared inside a subroutine. 2.4.2 Scope A symbol's scope is the portion of the program where that symbol's declaration is in effect, i.e. where that symbol is visible. Euphoria has many pre-defined procedures, functions and types. These are defined automatically at the start of any program. The Euphoria editor shows them in magenta. These pre-defined names are not reserved. You can override them with your own variables or routines. Every user-defined symbol must be declared before it is used. You can read a Euphoria program from beginning to end without encountering any user-defined variables or routines that haven't been defined yet. It is possible to call a routine that comes later in the source, but you must use the special functions, routine_id(), and either call_func() or call_proc() to do it. See Part II - Library Routines - Dynamic Calls. Procedures, functions and types can call themselves recursively. Mutual recursion, where routine A calls routine B which directly or indirectly calls routine A, requires the routine_id() mechanism. A symbol is defined from the point where it is declared to the end of its scope. The scope of a variable declared inside a procedure or function (a private variable) ends at the end of the procedure or function. The scope of all other variables, constants, procedures, functions and types ends at the end of the source file in which they are declared and they are referred to as local, unless the keyword global precedes their declaration, in which case their scope extends indefinitely. When you include a Euphoria file in a main file (see 2.6 Special Top-Level Statements), only the variables and routines declared using the global keyword are accessible or even visible to the main file. The other, non-global, declarations in the included file are forgotten at the end of the included file, and you will get an error message, "not declared", if you try to use them in the main file. Symbols marked as global can be used externally. All other symbols can only be used internally within their own file. This information is helpful when maintaining or enhancing the file, or when learning how to use the file. You can make changes to the internal routines and variables, without having to examine other files, or notify other users of the include file. Sometimes, when using include files developed by others, you will encounter a naming conflict. One of the include file authors has used the same name for a global symbol as one of the other authors. If you have the source, you can simply edit one of the include files to correct the problem, but then you'd have repeat this process whenever a new version of the include file was released. Euphoria has a simpler way to solve this. Using an extension to the include statement, you can say for example: ``` include johns_file.e as john include bills_file.e as bill john:x += 1 bill:x += 2 ``` In this case, the variable x was declared in two different files, and you want to refer to both variables in your file. Using the namespace identifier of either john or bill, you can attach a prefix to x to indicate which x you are referring to. We sometimes say that john refers to one namespace, while bill refers to another distinct namespace. You can attach a namespace identifier to any user-defined variable, constant, procedure or function. You can do it to solve a conflict, or simply to make things clearer. A namespace identifier has local scope. It is known only within the file that declares it, i.e. the file that contains the include statement. Different files might define different namespace identifiers to refer to the same included file. Euphoria encourages you to restrict the scope of symbols. If all symbols were automatically global to the whole program, you might have a lot of naming conflicts, especially in a large program consisting of files written by many different programmers. A naming conflict might cause a compiler error message, or it could lead to a very subtle bug, where different parts of a program accidentally modify the same variable without being aware of it. Try to use the most restrictive scope that you can. Make variables private to one routine where possible, and where that isn't possible, make them local to a file, rather than global to the whole program. When Euphoria looks up the declaration of a symbol, it first checks the current routine, then the current file, then globals in other files. Symbols that are more local will override symbols that are more global. At the end of the scope of the local symbol, the more global symbol will be visible again. If multiple global symbols are found, Euphoria will look to see if there is only one matching symbol that resides in a file that was directly or indirectly included from the file which is attempting to use the symbol. This way, third party libraries can be used in the same application, even if they have conflicting symbols. The application code that uses the libraries will have to use explicit namespaces to deconflict the symbols, but the libraries themselves will not need to be edited. Using a namespace identifier with a symbol name causes Euphoria to resolve the symbol as though it were being used from the file associated with the namespace. This means that a namespace could be associated with the main include file of a multi-file library, and using the namespace associated with the main file will allow deconfliction of all symbols within the library from other symbols in files independent of the library. Of course, if there are conflicting symbols within the library that require the use of namespace identifiers from within the library itself, users of the library will not be able to deconflict those symbols with the namespace associated with the main file of the library. Constant declarations must be outside of any subroutine. Constants can be global or local, but not private. Variable declarations inside a subroutine must all appear at the beginning, before the executable statements of the subroutine. Declarations at the top level, outside of any subroutine, must not be nested inside a loop or if-statement. The controlling variable used in a for-loop is special. It is automatically declared at the beginning of the loop, and its scope ends at the end of the for-loop. If the loop is inside a function or procedure, the loop variable is a private variable and may not have the same name as any other private variable. When the loop is at the top level, outside of any function or procedure, the loop variable is a local variable and may not have the same name as any other local variable in that file. You can use the same name in many different for-loops as long as the loops aren't nested. You do not declare loop variables as you would other variables. The range of values specified in the for statement defines the legal values of the loop variable - specifying a type would be redundant and is not allowed. 2.4.3 Specifying the Type of a Variable So far you've already seen some examples of variable types but now we will define types more precisely. Variable declarations have a type name followed by a list of the variables being declared. For example, ``` object a global integer x, y, z procedure fred(sequence q, sequence r) ``` The types: object, sequence, atom and integer are predefined. Variables of type object may take on any value. Those declared with type sequence must always be sequences. Those declared with type atom must always be atoms. Those declared with type integer must be atoms with integer values from -1073741824 to +1073741823 inclusive. You can perform exact calculations on larger integer values, up to about 15 decimal digits, but declare them as atom, rather than integer. Note: In a procedure or function parameter list like the one for fred() above, a type name may only be followed by a single parameter name. Performance Note: Calculations using variables declared as integer will usually be somewhat faster than calculations involving variables declared as atom. If your machine has floating-point hardware, Euphoria will use it to manipulate atoms that aren't representable as integers. If your machine doesn't have floating-point hardware, Euphoria will call software floating-point arithmetic routines contained in ex.exe (or in Windows). You can force ex.exe to bypass any floating-point hardware, by setting an environment variable: ``` SET NO87=1 ``` The slower software routines will be used, but this could be of some advantage if you are worried about the floating-point bug in some early Pentium chips. To augment the predefined types, you can create user-defined types. All you have to do is define a single-parameter function, but declare it with type ... end type instead of function ... end function. For example, ``` type hour(integer x) return x >= 0 and x <= 23 end type hour h1, h2 h1 = 10 -- ok h2 = 25 -- error! program aborts with a message ``` Variables h1 and h2 can only be assigned integer values in the range 0 to 23 inclusive. After each assignment to h1 or h2 the interpreter will call hour(), passing the new value. The value will first be checked to see if it is an integer (because of "integer x"). If it is, the return statement will be executed to test the value of x (i.e. the new value of h1 or h2). If hour() returns true, execution continues normally. If hour() returns false then the program is aborted with a suitable diagnostic message. "hour" can be used to declare subroutine parameters as well: ``` procedure set_time(hour h) ``` set_time() can only be called with a reasonable value for parameter h, otherwise the program will abort with a message. A variable's type will be checked after each assignment to the variable (except where the compiler can predetermine that a check will not be necessary), and the program will terminate immediately if the type function returns false. Subroutine parameter types are checked each time that the subroutine is called. This checking guarantees that a variable can never have a value that does not belong to the type of that variable. Unlike other languages, the type of a variable does not affect any calculations on the variable. Only the value of the variable matters in an expression. The type just serves as an error check to prevent any "corruption" of the variable. User-defined types can catch unexpected logical errors in your program. They are not designed to catch or correct user input errors. Type checking can be turned off or on between subroutines using the with type_check or without type_check special statements. It is initially on by default. Note to Benchmarkers: When comparing the speed of Euphoria programs against programs written in other languages, you should specify without type_check at the top of the file. This gives Euphoria permission to skip run-time type checks, thereby saving some execution time. All other checks are still performed, e.g. subscript checking, uninitialized variable checking etc. Even when you turn off type checking, Euphoria reserves the right to make checks at strategic places, since this can actually allow it to run your program faster in many cases. So you may still get a type check failure even when you have turned off type checking. Whether type checking is on or off, you will never get a machine-level exception. You will always get a meaningful message from Euphoria when something goes wrong. (This might not be the case when you poke directly into memory, or call routines written in C or machine code.) Euphoria's method of defining types is simpler than what you will find in other languages, yet Euphoria provides the programmer with greater flexibility in defining the legal values for a type of data. Any algorithm can be used to include or exclude values. You can even declare a variable to be of type object which will allow it to take on any value. Routines can be written to work with very specific types, or very general types. For many programs, there is little advantage in defining new types, and you may wish to stick with the four predefined types. Unlike other languages, Euphoria's type mechanism is optional. You don't need it to create a program. However, for larger programs, strict type definitions can aid the process of debugging. Logic errors are caught close to their source and are not allowed to propagate in subtle ways through the rest of the program. Furthermore, it is easier to reason about the misbehavior of a section of code when you are guaranteed that the variables involved always had a legal value, if not the desired value. Types also provide meaningful, machine-checkable documentation about your program, making it easier for you or others to understand your code at a later date. Combined with the subscript checking, uninitialized variable checking, and other checking that is always present, strict run-time type checking makes debugging much easier in Euphoria than in most other languages. It also increases the reliability of the final program since many latent bugs that would have survived the testing phase in other languages will have been caught by Euphoria. Anecdote 1: In porting a large C program to Euphoria, a number of latent bugs were discovered. Although this C program was believed to be totally "correct", we found: a situation where an uninitialized variable was being read; a place where element number "-1" of an array was routinely written and read; and a situation where something was written just off the screen. These problems resulted in errors that weren't easily visible to a casual observer, so they had survived testing of the C code. Anecdote 2: The Quick Sort algorithm presented on page 117 of Writing Efficient Programs by Jon Bentley has a subscript error! The algorithm will sometimes read the element just before the beginning of the array to be sorted, and will sometimes read the element just after the end of the array. Whatever garbage is read, the algorithm will still work - this is probably why the bug was never caught. But what if there isn't any (virtual) memory just before or just after the array? Bentley later modifies the algorithm such that this bug goes away -- but he presented this version as being correct. Even the experts need subscript checking! Performance Note: When typical user-defined types are used extensively, type checking adds only 20 to 40 percent to execution time. Leave it on unless you really need the extra speed. You might also consider turning it off for just a few heavily-executed routines. Profiling can help with this decision. 2.5 Statements The following kinds of executable statements are available: Semicolons are not used in Euphoria, but you are free to put as many statements as you like on one line, or to split a single statement across many lines. You may not split a statement in the middle of an identifier, string, number or keyword. 2.5.1 assignment statement An assignment statement assigns the value of an expression to a simple variable, or to a subscript or slice of a variable. e.g. ``` x = a + b y[i] = y[i] + 1 y[i..j] = {1, 2, 3} ``` The previous value of the variable, or element(s) of the subscripted or sliced variable are discarded. For example, suppose x was a 1000-element sequence that we had initialized with: ``` object x x = repeat(0, 1000) -- a sequence of 1000 zeros ``` and then later we assigned an atom to x with: ``` x = 7 ``` This is perfectly legal since x is declared as an object. The previous value of x, namely the 1000-element sequence, would simply disappear. Actually, the space consumed by the 1000-element sequence will be automatically recycled due to Euphoria's dynamic storage allocation. Note that the equals symbol '=' is used for both assignment and for equality testing. There is never any confusion because an assignment in Euphoria is a statement only, it can't be used as an expression (as in C). Assignment with Operator Euphoria also provides some additional forms of the assignment statement. To save typing, and to make your code a bit neater, you can combine assignment with one of the operators:       +   -   /   *   & For example, instead of saying: ``` mylongvarname = mylongvarname + 1 ``` You can say: ``` mylongvarname += 1 ``` Instead of saying: ``` galaxy[q_row][q_col][q_size] = galaxy[q_row][q_col][q_size] * 10 ``` You can say: ``` galaxy[q_row][q_col][q_size] *= 10 ``` and instead of saying: ``` accounts[start..finish] = accounts[start..finish] / 10 ``` You can say: ``` accounts[start..finish] /= 10 ``` In general, whenever you have an assignment of the form: ``` left-hand-side = left-hand-side op expression ``` You can say: ``` left-hand-side op= expression ``` where op is one of:    +   -   *   /   & When the left-hand-side contains multiple subscripts/slices, the op= form will usually execute faster than the longer form. When you get used to it, you may find the op= form to be slightly more readable than the long form, since you don't have to visually compare the left-hand-side against the copy of itself on the right side. 2.5.2 procedure call A procedure call starts execution of a procedure, passing it an optional list of argument values. e.g. ``` plot(x, 23) ``` 2.5.3 if statement An if statement tests a condition to see if it is 0 (false) or non-zero (true) and then executes the appropriate series of statements. There may be optional elsif and else clauses. e.g. ``` if a < b then x = 1 end if if a = 9 and find(0, s) then x = 4 y = 5 else z = 8 end if if char = 'a' then x = 1 elsif char = 'b' or char = 'B' then x = 2 elsif char = 'c' then x = 3 else x = -1 end if ``` Notice that elsif is a contraction of else if, but it's cleaner because it doesn't require an end if to go with it. There is just one end if for the entire if statement, even when there are many elsif's contained in it. The if and elsif conditions are tested using short-circuit evaluation. 2.5.4 while statement A while statement tests a condition to see if it is non-zero (true), and while it is true a loop is executed. e.g. ``` while x > 0 do a = a * 2 x = x - 1 end while ``` Short-Circuit Evaluation When the condition tested by if, elsif, or while contains and or or operators, short-circuit evaluation will be used. For example, ``` if a < 0 and b > 0 then ... ``` If a < 0 is false, then Euphoria will not bother to test if b is greater than 0. It will assume that the overall result is false. Similarly, ``` if a < 0 or b > 0 then ... ``` if a < 0 is true, then Euphoria will immediately decide that the result true, without testing the value of b. In general, whenever we have a condition of the form: ``` A and B ``` where A and B can be any two expressions, Euphoria will take a short-cut when A is false and immediately make the overall result false, without even looking at expression B. Similarly, with: ``` A or B ``` when A is true, Euphoria will skip the evaluation of expression B, and declare the result to be true. If the expression B contains a call to a function, and that function has possible side-effects, i.e. it might do more than just return a value, you will get a compile-time warning. Older versions (pre-2.1) of Euphoria did not use short-circuit evaluation, and it's possible that some old code will no longer work correctly, although a search of the Euphoria archives did not turn up any programs that depend on side-effects in this way. The expression, B, could contain something that would normally cause a run-time error. If Euphoria skips the evaluation of B, the error will not be discovered. For instance: ``` if x != 0 and 1/x > 10 then -- divide by zero error avoided while 1 or {1,2,3,4,5} do -- illegal sequence result avoided ``` B could even contain uninitialized variables, out-of-bounds subscripts etc. This may look like sloppy coding, but in fact it often allows you to write something in a simpler and more readable way. For instance: ``` if atom(x) or length(x)=1 then ``` Without short-circuiting, you would have a problem when x was an atom, since length is not defined for atoms. With short-circuiting, length(x) will only be checked when x is a sequence. Similarly: ``` -- find 'a' or 'A' in s i = 1 while i <= length(s) and s[i] != 'a' and s[i] != 'A' do i += 1 end while ``` In this loop the variable i might eventually become greater than length(s). Without short-circuit evaluation, a subscript out-of-bounds error will occur when s[i] is evaluated on the final iteration. With short-circuiting, the loop will terminate immediately when i <= length(s) becomes false. Euphoria will not evaluate s[i] != 'a' and will not evaluate s[i] != 'A'. No subscript error will occur. Short-circuit evaluation of and and or takes place for if, elsif and while conditions only. It is not used in other contexts. For example, the assignment statement: ``` x = 1 or {1,2,3,4,5} -- x should be set to {1,1,1,1,1} ``` If short-circuiting were used here, we would set x to 1, and not even look at {1,2,3,4,5}. This would be wrong. Short-circuiting can be used in if/elsif/while conditions because we only care if the result is true or false, and conditions are required to produce an atom as a result. 2.5.5 for statement A for statement sets up a special loop with a controlling loop variable that runs from an initial value up or down to some final value. e.g. ``` for i = 1 to 10 do ? i -- ? is a short form for print() end for -- fractional numbers allowed too for i = 10.0 to 20.5 by 0.3 do for j = 20 to 10 by -2 do -- counting down ? {i, j} end for end for ``` The loop variable is declared automatically and exists until the end of the loop. Outside of the loop the variable has no value and is not even declared. If you need its final value, copy it into another variable before leaving the loop. The compiler will not allow any assignments to a loop variable. The initial value, loop limit and increment must all be atoms. If no increment is specified then +1 is assumed. The limit and increment values are established when the loop is entered, and are not affected by anything that happens during the execution of the loop. See also the scope of the loop variable in 2.4.2 Scope. 2.5.6 return statement A return statement returns immediately from a subroutine. If the subroutine is a function or type then a value must also be returned. e.g. ``` return return {50, "FRED", {}} ``` 2.5.7 exit statement An exit statement may appear inside a while-loop or a for-loop. It causes immediate termination of the loop, with control passing to the first statement after the loop. e.g. ``` for i = 1 to 100 do if a[i] = x then location = i exit end if end for ``` It is also quite common to see something like this: ``` constant TRUE = 1 while TRUE do ... if some_condition then exit end if ... end while ``` i.e. an "infinite" while-loop that actually terminates via an exit statement at some arbitrary point in the body of the loop. Performance Note: Euphoria optimizes this type of loop. At run-time, no test is performed at the top of the loop. There's just a simple unconditional jump from end while back to the first statement inside the loop. With ex.exe, if you happen to create a real infinite loop, with no input/output taking place, there is no easy way to stop it. You will have to type Control-Alt-Delete to either reboot, or (under Windows) terminate your DOS prompt session. If the program had files open for writing, it would be advisable to run scandisk to check your file system integrity. Only when your program is waiting for keyboard input, will control-c abort the program (unless allow_break(0) was used). With exw.exe or exu, control-c will always stop your program immediately. 2.6 Special Top-Level Statements Before any of your statements are executed, the Euphoria front-end quickly reads your entire program. All statements are syntax checked and converted to a low-level intermediate language (IL). The interpreter immediately executes the IL. The translator converts the IL to C. The binder/shrouder saves the IL on disk for later execution. These three tools all share the same front-end (written in Euphoria). If your program contains only routine and variable declarations, but no top-level executable statements, then nothing will happen when you run it (other than syntax checking). You need a top-level statement to call your main routine (see 1.1 Example Program). It's quite possible to have a program with nothing but top-level executable statements and no routines. For example you might want to use Euphoria as a simple calculator, typing just a few print (or ?) statements into a file, and then executing it. As we have seen, you can use any Euphoria statement, including for-loops, while-loops, if statements etc. (but not return), at the top level i.e. outside of any function or procedure. In addition, the following special statements may only appear at the top level: include with / without 2.6.1 include When you write a large program it is often helpful to break it up into logically separate files, by using include statements. Sometimes you will want to reuse some code that you have previously written, or that someone else has written. Rather than copy this code into your main program, you can use an include statement to refer to the file containing the code. The first form of the include statement is: include filename This reads in (compiles) a Euphoria source file. Some Examples: ``` include graphics.e ``` ``` include \mylib\myroutines.e ``` Any top-level code in the included file will be executed. Any global symbols that have already been defined in the main file will be visible in the included file. N.B. Only those symbols defined as global in the included file will be visible (accessible) in the remainder of the program. If an absolute filename is given, Euphoria will open it and start parsing it. When a relative filename is given, Euphoria will try to open the file relative to the following directories, in the following order: The directory containing the current source file. i.e. the source file that contains the include statement that is being processed.. The directory containing the main file given on the interpreter, translator or binder command-line. If you've defined an environment variable, EUINC, Euphoria will check each directory listed in EUINC (from left to right). EUINC should be a list of directories, separated by semicolons (colons on Linux / FreeBSD), similar in form to your PATH variable. EUINC can be added to your set of Linux / FreeBSD or DOS / Windows environment variables. (Via Control Panel / Performance & Maintenance / System / Advanced on XP, or AUTOEXEC.BAT on older versions of Windows). e.g.       SET EUINC=C:\EU\MYFILES;C:\EU\WIN32LIB EUINC lets you organize your include files according to application areas, and avoid adding numerous unrelated files to euphoria\include. Finally, if it still hasn't found the file, it will look in euphoria\include. This directory contains the standard Euphoria include files. The environment variable EUDIR tells Euphoria where to find your euphoria directory. An included file can include other files. In fact, you can "nest" included files up to 30 levels deep. Include file names typically end in .e, or sometimes .ew or .eu (when they are intended for use with Windows or Linux). This is just a convention. It is not required. If your filename (or path) contains blanks, you must enclose it in double-quotes, otherwise quotes are optional. Also, be sure to double-up your backslashes. For example: ``` include "c:\\program files\\myfile.e" ``` Other than possibly defining a new namespace identifier (see below), an include statement will be quietly ignored if the same file has already been included. An include statement must be written on a line by itself. Only a comment can appear after it on the same line. The second form of the include statement is: include filename as namespace_identifier This is just like the simple include, but it also defines a namespace identifier that can be attached to global symbols in the included file that you want to refer to in the main file. This might be necessary to disambiguate references to those symbols, or you might feel that it makes your code more readable. See Scope Rules for more. 2.6.2 with / without These special statements affect the way that Euphoria translates your program into internal form. They are not meant to change the logic of your program, but they may affect the diagnostic information that you get from running your program. See 3. Debugging and Profiling for more information. with This turns on one of the options: profile, profile_time, trace, warning or type_check . Options warning and type_check are initially on, while profile, profile_time and trace are initially off. Any warnings that are issued will appear on your screen after your program has finished execution. Warnings indicate very minor problems. A warning will never stop your program from executing. without This turns off one of the above options. There is also a rarely-used special with option where a code number appears after with. In previous releases this code was used by RDS to make a file exempt from adding to the statement count in the old "Public Domain" Edition. You can select any combination of settings, and you can change the settings, but the changes must occur between subroutines, not within a subroutine. The only exception is that you can only turn on one type of profiling for a given run of your program. An included file inherits the with/without settings in effect at the point where it is included. An included file can change these settings, but they will revert back to their original state at the end of the included file. For instance, an included file might turn off warnings for itself and (initially) for any files that it includes, but this will not turn off warnings for the main file.   ... continue 3. Debugging and Profiling