attributes on expressions, not name

attributes on expressions, not name

[Mehdi Saada]

why aren't attributes allowed on expressions ?
Like LINE'Length, assuming LINE is a function returning STRING.
That limitation is PAINFUL.
The type of an expression is always static, even when it is a class-wide type, an attribute defined for the type of that expression, will never fail. At least I can't see any case. Please tell me if I'm wrong.

Re: attributes on expressions, not name

[Niklas Holsti]

On 18-02-28 18:57 , Mehdi Saada wrote:
> why aren't attributes allowed on expressions ?
> Like LINE'Length, assuming LINE is a function returning STRING.

There is no such limitation.

RM 4.1.4: attribute_reference ::= prefix'attribute_designator

RM 4.1: prefix ::= name ...

RM 4.1; name ::= ... function_call ...

> The type of an expression is always static,

The type of the value returned by a function call is known only after 
compile-time overload resolution, which depends on the context of the 
expression. Applying an attribute to (the result of) a function call 
does not always provide enough context to resolve overloading.

In your example, there could be several different functions called LINE, 
some returning types to which the Length attribute could apply, and some 
returning types to which it does not apply. It is not clear which of 
these functions should be called.

If I have one function LINE return String, and another LINE return 
Integer, and try to compute LINE'Length, GNAT tells me:

func_attr.adb:22:41: ambiguous prefix for "Length" attribute

Is that the error message you get?

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .

Re: attributes on expressions, not name

[Alejandro R. Mosteo]

On 28/02/18 18:55, Niklas Holsti wrote:
> 
> If I have one function LINE return String, and another LINE return 
> Integer, and try to compute LINE'Length, GNAT tells me:
> 
> func_attr.adb:22:41: ambiguous prefix for "Length" attribute

Recently in a similar case, GNAT even suggested to me to disambiguate 
like so:

String'(LINE)'Length

Re: attributes on expressions, not name

[Mehdi Saada]

It said the same 10 minutes after. Saved me a tour in the RM.
I did solve my case with a qualified expression.