10 2 A TINY IMPERATIVE PROGRAMMING LANGUAGE
| input
Expressions Exp include integer literals Int and variables (identifiers) Id. The
input
expression reads an integer from the input stream. The comparison
operators yield 0 for false and 1 for true. Function calls, pointer operations, and
record operations will be added later.
Statements
The simple statements Stm are familiar:
Stm → Id = Exp ;
| output Exp ;
| Stm Stm
|
| if ( Exp ) { Stm }
else { Stm }
?
| while ( Exp ) { Stm }
We use the notation
. . .
?
to indicate optional parts. In the conditions we
interpret 0 as false and all other values as true. The
output
statement writes an
integer value to the output stream.
Functions
A function declaration F contains a function name, a list of parameters, local
variable declarations, a body statement, and a return expression:
Fun → Id ( Id , . . . , Id ) {
var Id , . . . , Id ;
?
Stm return Exp; }
Function names and parameters are identifiers, like variables. The
var
block
declares a collection of uninitialized local variables. Function calls are an extra
kind of expression:
Exp → . .. | Id ( Exp,. . . ,Exp )
We sometimes treat var blocks and return instructions as statements.
Functions as Values
We also allow functions as first-class values. The name of a function can be used
as a kind of variable that refers to the function, and such function values can be
assigned to ordinary variables, passed as arguments to functions, and returned
from functions.
We add a generalized form of function calls (sometimes called computed or
indirect function calls, in contrast to the simple direct calls described earlier):
Exp → . .. | Exp ( Exp , .. ., Exp )
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