On 23 Μαϊ 2007, at 3:05 ΜΜ, Tom Verhoeff wrote:
On Wed, May 23, 2007 at 11:36:20AM +0300, Christos Chryssochoidis
wrote:
It *is* allowed. The only trick is that you have to declare a
type for
your dynamic array, i.e. you have to write something like
Yes, I have noticed that if you first wrap a dynamic array in a type
definition then you can declare a function returning such an array,
but it seemed to me as an unnecessary complication of things and as
an inconsistency in the language. However, the distinction between
dynamic and open arrays that D.Mantione says, explains this
complication.
It is not a trick, but has to do with type compatibility issues
(and history, I guess).
If you have
function f(...): SomeType;
then the result of calling f needs to go somewhere. Either it
"disappears" as an intermediate result in a larger expression, or you
assign it to a variable (or such). In case it is a subexpression,
the result of f acts as a parameter of an enclosing function or
operator.
while f(...) operator ... do ...
if g( f(...) ) then ...
v := f(...)
The receiving parameter or variable needs to have a type "compatible"
with SomeType. If SomeType is a type expression, then you cannot
declare
a parameter or variable of the same type by copying the type
expression.
In Pascal,
var
a: array [ Char ] of Integer;
b: array [ Char ] of Integer;
function g(p: array [ Char ] of Integer);
function f(c: Char; i: Integer): array [ Char ] of Integer;
does not (automatically) make a and b have the same type (in some
cases automatic conversions etc. will give you some kind of
compatibility).
For that (partly historic) reason, parameter types and return types
must
be type names and not type expressions.
Of course, there situations where you can argue that it works without
using type names. E.g. if you return a record type, one way of using
it would be to select a field:
function f(...): record x, y: Integer end;
if f(...).x = 0 then ...
The "operation" of field selection works on arbitrary record types.
Insisting on a type name seems unneeded. I.e. the typing of
the "parameters" of . is not so strict; it occurs in a context
record . fieldname_of_this_record_type
The record is not a parameter of . in the usual sense.
I haven't checked the FPC documentation on all this.
Tom
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E-MAIL: T.Verhoeff @ TUE.NL | Dept. of Math. & Comp. Science
PHONE: +31 40 247 41 25 | Technische Universiteit Eindhoven
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I see. If I understand well, Pascal uses some sort of declaration
equivalence for types.
Christos
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