| forward | subprogram declaration |
| Syntax | A forwardDeclaration is:
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| Description | A procedure or function is declared to be forward when you want to define its header but not its body. This is the case when one procedure or function calls another, which in turn calls the first; this situation is called mutual recursion. The use of forward is necessary in this case, because every item must be declared before it can be used.
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| Example | This example program evaluates an input expression e of the form t { + t } where t is of the form p { * p } and p is of the form (e ) or an explicit real expression. For example, the value of 1.5 + 3.0 * (0.5 + 1.5) halt is 7.5.
var token : string
forward procedure expn ( var eValue : real )
forward procedure term ( var tValue : real )
forward procedure primary ( var pValue: real )
body procedure expn
var nextValue : real
term ( eValue ) % Evaluate t
loop % Evaluate { + t}
exit when token not= "+"
get token
term ( nextValue )
eValue := eValue + nextValue
end loop
end expn
body procedure term
var nextValue : real
primary (tValue ) % Evaluate p
loop % Evaluate { * p}
exit when token not= "*"
get token
primary ( nextValue )
tValue := tValue * nextValue
end loop
end term
body procedure primary
if token = "(" then
get token
expn ( pValue ) % Evaluate (e)
assert token = ")"
else % Evaluate "explicit real"
pValue := strreal ( token )
end if
get token
end primary
get token % Start by reading first token
var answer : real
expn ( answer ) % Scan and evaluate input expression
put "Answer is ", answer
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| Details | Following a forward procedure or function declaration, the body of the procedure must be given at the same level (in the same sequence of statements and declarations as the forward declaration). This is the only use of the keyword body. See also body. Any procedure or function that is declared using forward requires an import list. In this list, imported procedures or functions that have not yet appeared must be listed as forward. For example, the import list for expn is import forward term … Before a procedure or function can be called, before its body appears, and before it can be passed as a parameter, its header as well as headers of procedures or functions imported directly or indirectly by it must have appeared. The keyword forward is also used in collection and type declarations.
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| See also | collection and type declarations.
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