Thursday, March 13, 2008

Procedurally-typed expressions redux

A few days ago I blogged about some issues that the Delphi parser has with certain constructs. A little discussion ensued in the comments, so I think I want to clarify what I meant a little.

Specifically, there were two problems I pointed out:

  1. Calling procedurally-typed values in non-trivial expressions
  2. Creating procedurally-typed values from no-arg procedures (and functions and methods etc.) that appear in slightly non-trivial expressions.

The first problem can be solved by interpreting the appearance of an argument list (introduced by '(') in an expression as an attempt to call what is effectively the left hand side of the '(' operator. The second problem can be solved by more deeply analysing the expression tree during argument processing so that it isn't confused by something as simple as an extra set of parentheses. The first problem is more pressing as it's affecting certain features I'm trying to get into the product.

Nothing will be imposed. There will be no breaking changes in syntax or semantics, if it can possibly be helped. All that should change is that previously invalid code becomes valid.

While there is an ambiguity when looking at some hypothetical function to a function to a function, where, when calling f()(), it might appear unclear from the rules which function is being called, this syntax is currently not valid at all. Currently, the first set of parentheses is parsed as part of the first call to f, but the attempt to call the return value (of type function to a function) will fail. Extending this so that the second call succeeds shouldn't be problematic, because there's a simple rule: the '()' on a no-arg function is optional, but it will be parsed if it's found. It can't be "delayed", such that the raw 'f' is interpreted as a function call and then the '()' applied to the return value. The parser eats the tokens when it sees them and it has a rule for matching them.

The further testing I did for this post exposed another problem, potentially more severe than the others - it's a type hole:

{$apptype console}
{$T+} // you'd like typed-@ to help you here, but it doesn't...

 TF = function: Integer;
 PF = ^TF;

function MyF: Integer;
 Result := 42;

// P is separate procedure so 'f' stands out as a stack variable
procedure P;
 f: TF;
 x: PF;
 // use absolute to get around oddities of procedural types
 f1: Integer absolute f;
 x1: Integer absolute x;
 Writeln('MyF is at ', Integer(@MyF));
 Writeln('f is at ', Integer(@@f));
 f := MyF;
 Writeln('f is pointing to ', f1);
 x := @f;
 Writeln('x should be pointing to f, but is pointing to ', x1);
 // The following line doesn't do what you'd expect: it crashes.
 // Writeln(x^);

Unhappily enough, this prints out the following on my machine, with no warnings or errors during compilation:
MyF is at 4211536
f is at 1245092
f is pointing to 4211536
x should be pointing to f, but is pointing to 4211536
The line commented out does in fact crash.

Update: Craig in the comments asks why it's wrong. There are at least two things that could be wrong, and one of the unfortunate problems here is that there's no standard for Delphi beyond what the compiler currently does, so we can't say for sure. Either the '@f' when assigned to x should take the address of f rather than just inhibit calling the function pointer, or it should result in a compiler error - particularly as typed-@ operators are turned on here. The commented out line, x^, ought to call the function pointer being pointed to by x, but it doesn't, since it's simply pointing to the function 'MyF', rather than the function pointer 'f'.


Craig said...

OK, that looks wrong, but why is it wrong? Is it just a compiler bug, or is the compiler trying to do something non-obvious here?


Xepol said...

Some of the stranger syntax is like

@Pointer := assignment;

But there is still the much larger bug of being able to use the @ operation on an overloaded function - it does not always point at the function you OR the compiler expects and calls can fail horribly.


function a : Integer; Overload;
Begin Result := 42; End;
Function a( S: string) : string; Overload;
Begin Result := S+' fourty two'; End;

Type ta=Function : Integer;

aa : ta;
aa := @a;

RemObject's scripting uses the @function and if you run into overloaded functions, even if you give them seperate names on the script side, bad, bad things can and do happen.