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digitalmars.D.learn - Const struct assign error messages

reply bearophile <bearophileHUGS lycos.com> writes:
This is a little wrong D2 program:


void bar(const int data) {
    auto d = data;
    d = data;
}
void main() {}


With the error message DMD gives it's easy to understand the error and fix it:
test.d(3): Error: variable test.bar.d cannot modify const


This is a similar program that uses a Tuple instead of an int:

import std.typecons;
alias Tuple!(int) Foo;
void bar(const Foo data) {
    auto d = data;
    d = data;
}
void main() {}


But now the errors given are not so easy to understand:
test.d(5): Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if
(isTuple!(R)) does not match any function template declaration
test.d(5): Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if
(isTuple!(R)) cannot deduce template function from argument types
!()(const(Tuple!(int)))


Being that "d" a const Foo struct, that you can never be assigned to, can't DMD
give a nicer error message, like in the first program?

Bye,
bearophile
Mar 04 2011
parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Friday, March 04, 2011 15:19:22 bearophile wrote:
 This is a little wrong D2 program:
 
 
 void bar(const int data) {
     auto d = data;
     d = data;
 }
 void main() {}
 
 
 With the error message DMD gives it's easy to understand the error and fix
 it: test.d(3): Error: variable test.bar.d cannot modify const
 
 
 This is a similar program that uses a Tuple instead of an int:
 
 import std.typecons;
 alias Tuple!(int) Foo;
 void bar(const Foo data) {
     auto d = data;
     d = data;
 }
 void main() {}
 
 
 But now the errors given are not so easy to understand:
 test.d(5): Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if
 (isTuple!(R)) does not match any function template declaration test.d(5):
 Error: template std.typecons.Tuple!(int).Tuple.opAssign(R) if
 (isTuple!(R)) cannot deduce template function from argument types
 !()(const(Tuple!(int)))
 
 
 Being that "d" a const Foo struct, that you can never be assigned to, can't
 DMD give a nicer error message, like in the first program?

Well, you just added a template into the mix. That complicates things considerably. It's going to have to actually instantiate the templated opAssign before it will have code to compare similar to the first situation. And since the template constraint is failing, the template doesn't get instantiated and so you don't get the nice error. I don't think that there's anything that the compiler can really do about that. If you adjusted the template constraint, you'd likely just get an error in compiling opAssign and that error would be in std.typecons, not your code, so it would make it lookng like std.typecons was broken rather than your code, and it would probably be much harder to track down, since I don't think that it would tell you which line of your code is causing the problem. So, while I can see while you'd want a better error message, I don't think that you can do it with templated function. The way templates work just doesn't lend itself to doing what you want. You'd have to be able to instantiate an invalid template, which you obviously can't do. - Jonathan M Davis
Mar 04 2011
parent bearophile <bearophileHUGS lycos.com> writes:
Jonathan M Davis:

 Well, you just added a template into the mix.

Right. The opAssign of Tuple is: void opAssign(R)(R rhs) if (isTuple!R)
And since the template constraint is failing,<

This little test program shows that the template constraint of opAssign isn't needed to produce the same two error messages: struct Foo { int x; void opAssign(R)(R other) { // x = other.x; } } void bar(const Foo data) { auto d = data; d = data; // line 9 } void main() {} test.d(9): Error: template test.Foo.opAssign(R) does not match any function template declaration test.d(9): Error: template test.Foo.opAssign(R) cannot deduce template function from argument types !()(const(Foo)) The semantics of opAssign() is to assign something to the struct, even if opAssign() is templated. So I think the compiler has enough information to give a nicer error message if a struct with opAssign is const. Thank you for your answers, bye, bearophile
Mar 04 2011