Compila 22 language specification

In the description of the grammar later, we use capital letters for non-terminals. As meta-symbols for the grammar, we use the following:

   ->,  |,  (,  ), {,  },  [,  ], "  

Commas in the previous line are used as ``meta-meta symbols'' in the enumeration of the meta-symbols.

When writing down the grammar in some variant of EBNF, {...} represents iteration of zero or more times, [...] represents optional clauses. Everything else, written as contiguous sequences, are terminal symbols. Those with only small letters are reserved keywords of the meta-language.

Note that terminal symbols of the Compila-language are written in ``string-quotes'' (i.e., with a " at the beginning and at the end) to distinguish them from symbols from the meta-language. Some specific terminal symbols are written in capitals, and without quotes. Those are

• NAME,
• INT_LITERAL,
• FLOAT_LITERAL and
• STRING_LITERAL.

See the following section about lexical aspects for what those terminal symbols represent.

• NAME must start with a letter, followed by a (possibly empty) sequence of numeric characters, letters, and underscore characters; the underscore is not allowed to occur at the end. Capital and small letters are considered different.
• All keywords of the languages are written in with lower-case letters. Keyword cannot be used for standard identifiers.
• INT_LITERAL contains one or more numeric characters.
• FLOAT_LITERAL contains one or more numeric characters, followed by a decimal point sign, which is followed by one or more numeric characters.
• STRING_LITERAL consists of a string of characters, enclosed in quotation marks ("). The string is not allowed to contain line shift, new-line, carriage return, or similar. The semantic value of a STRING_LITERAL is only the string itself, the quotation marks are not part of the string value itself.

Compila supports single line and multi-line comments.

1. Single-line comments start with // and the comment extends until the end of that line (as in, for instance, Java, C++, and most modern C-dialects).
2. Multi-line comments start with (* and end with *).

The latter form cannot be nested. The first one is allowed to be ``nested'' (in the sense that a commented out line can contain another // or the multi-line comment delimiters, which are then ignored).

The language has four built-in types and user-defined types:

• Built-in types
  1. floating point numbers ("float"),
  2. integers ("int"),
  3. strings ("string"), and
  4. booleans ("bool").
• User-defined types:
  1. Each (name of a) record represents a type.
  2. Reference types, representing references to elements of the specified types. The reference type constructor can be nested.

The language supports records. For people coming from Java/C++ etc., records can be seen as (very) simple form of classes, containing only instance variables as members, but support neither methods nor inheritance nor explicitly programmable constructors. ``Instance variables'' are more commonly called record fields or just fields when dealing with records. Records do support instantiation, here via the new keyword. Another aspect which resembles classes as in Java is that variables of record type contains either a pointer to an element (``object'') of that record type or the special pointer value null.¹

The language allows that variables can be declared to be of reference type ("pointers", if you will) by writing, e.g.,

 var x: ref(int);

Variables, like x, can be assigned values that are references, so, e.g., the following is allowed:

var y: int;
y := 42;
x := ref(y);

Correspondingly, one can ``follow'' a reference r by using deref(r), so that, given the previous definitions, the following is legal:

y := deref(x);

Expressions with deref can also be used as L-values, so that we can assign values to the location that they are pointing to, e.g.:

deref(x) := y;

See also later the swap procedure example later in the context of parameter passing.

The following productions in EBNF describe the syntax of the language. For precedences and associativity of various constructs, see later.

PROGRAM            -> "program" NAME "begin" [ DECL {";" DECL}]  "end"

DECL               -> VAR_DECL | PROC_DECL | REC_DECL

VAR_DECL           -> "var" NAME ":" TYPE [ ":=" EXP ] |  "var" NAME ":="  EXP 

PROC_DECL          -> "procedure" NAME 
                      "(" [ PARAMFIELD_DECL { "," PARAMFIELD_DECL } ] ")"
                      [ ":" TYPE ]
                      "begin" [[DECL{";" DECL}] "in"] STMT_LIST "end"
REC_DECL           -> "struct" NAME "{" [ PARAMFIELD_DECL 
                                          {";" PARAMFIELD_DECL }]  "}"
PARAMFIELD_DECL     -> NAME ":" TYPE


STMT_LIST          -> [STMT {";" STMT}]


EXP                -> EXP LOG_OP EXP
                    | "not" EXP
                    | EXP REL_OP EXP
                    | EXP ARITH_OP EXP
                    | LITERAL
                    | CALL_STMT
                    | "new" NAME
                    | VAR
                    | REF_VAR
                    | DEREF_VAR
                    | "(" EXP ")"

REF_VAR            -> "ref" "(" VAR ")"
DEREF_VAR          -> "deref" "(" VAR ")" | "deref" "(" DEREF_VAR ")"

VAR                -> NAME | EXP "." NAME

LOG_OP             -> "&&" | "||" 

REL_OP             -> "<" | "<=" | ">" | ">=" | "=" | "<>"

ARITH_OP           -> "+" | "-" | "*" | "/" | "^"

LITERAL            -> FLOAT_LITERAL | INT_LITERAL 
                    | STRING_LITERAL | BOOL_LITERAL | "null"

BOOL_LITERAL       ->  "true" | "false" 

STMT               -> ASSIGN_STMT
                    | IF_STMT
                    | WHILE_STMT
                    | RETURN_STMT
                    | CALL_STMT

ASSIGN_STMT        -> VAR ":=" EXP | DEREF_VAR ":=" EXP

IF_STMT            -> "if" EXP "then" { STMT_LIST } 
                       [ "else"  { STMT_LIST } ] "fi" 
WHILE_STMT         -> "while" EXP "do"  { STMT_LIST  } "od"

RETURN_STMT        -> "return" [ EXP ]

CALL_STMT          -> NAME "(" [ EXP { "," EXP } ] ")"

TYPE               -> "float" | "int" | "string" | "bool" | NAME
                    | "ref" "(" TYPE ")"

The precedence of the following constructs is ordered as follows (from lowest precedence to the highest):

1. ||
2. &&
3. not
4. All relational symbols
5. + and -
6. * and /
7. ^ (exponentiation)
8. . (``dot'', to access fields of a record)

• The binary operations ||, &&, +, -, *, and . are left-associative, but exponentiation ^ is right-associative.
• Relation symbols are non-associative. That means that for instance a < b + c < d is illegal.
• It's legal to write not not not b and it stands for (not (not (not b))).

The formal parameters are local variables in the procedure definition. When a procedure is being called, the values of the local parameters are copied into the corresponding formal parameters.²

The using occurence of an identifier (without a preceding dot) is bound in the common way to a declaration. This association of the use of an identifier to a declaration (``binding'') can be described informally as follows: Look through the block or scope which encloses the use-occurence of the identifer (where the block refers to the procedure body or program). The binding occurrence corresponding to the use occurence is the first declaration found in this way. If no binding occurence is found that way, the program is erroneous. Formal parameters count as declarations local to the procedure body.

Use occurences of a name preceded by a dot correspond to the clause EXP "." NAME in the production for the non-terminal VAR in the grammar. Those names are bound by looking at the type of EXP (which is required to be a record-type) and look up the field with name NAME in that record. It's an error, if EXP is not of record-type or else, there is not such field in that record,

• expressions: expressions need to be checked for type correctness in the obvious manner. The whole expression (if it type-checks) will thus carry a type.
• assignments: Both sides of an assignment must be of the same type. Note: it is allowed to assign to the formal parameters of a procedure. That applies to both call-by-value and call-by-reference parameters. Of course, the effect of an assignment in these two mechanisms is different.
• conditionals and while loop: the condition (i.e., expression) in the conditional construct must be of type bool. Same for the condition in the while loop.
• field selection:
  • the expression standing in front of a dot must be of record type.
  • the name standing after a dot are the name of a field/attribute of the record type of the expression in front of the dot. The type of the field selection expression (if it type checks) is the type as declared for the field of the record.

It is allowed to assign an expression of type int to a variable of type float. The inverse situation is not allowed. There is no type cast operator. If an arithmetic expression has at least one operand of type float, the operation is evaluated using floating point arithmetic and the result is of type float. Exponentiation is always considered done with floating point arithmetic and the result is of type float.

The language requires a tiny bit of type inference (so small that one might not even call type inference). Variables need to be declared, using the keyword var. In particular, variables must have a type at the point when they are declared. The declaration can be combined with a definition of an initial value of introduced variable (using the syntax like var x : int := 42). When provided with an initial expression, the type can be omitted (using a syntax like var x := 42), in which case an appropriate type needs to be inferred.

The logical operators && and || use so-called short-circuit evaluation. That means that if the value of the logical expression can be determined after one has evaluated the first part, only, the rest of the expression is not evaluated.