More precise error message for wrong lambda

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2014-11-04T10:53:56Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=13683

CC List

• greensunny12
• nick (@ntrel)

Description

This is wrong code:

class Foo {}
void main() {
    import std.algorithm: all;
    Foo[] data;
    assert(data.all!(f => f != null));
}


DMD 2.067alpha gives:

test.d(5,16): Error: template std.algorithm.all cannot deduce function from argument types !((f) => f != null)(Foo[]), candidates are:
..\dmd2\src\phobos\std\algorithm.d(12251,1):        std.algorithm.all(alias pred = "a")


The correct code needs "!is" instead of "!=":

assert(data.all!(f => f !is null));


The error message should tell me more precisely what's the problem.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2018-06-23T13:27:33Z

Changing to phobos bug because `all`'s constraint uses:

is(typeof(unaryFun!pred(range.front)))

Instead this could be made a static assert, so at least the user can be informed that the predicate is not callable with Foo.

[dlangBugzillaToGithub]

greensunny12 commented on 2018-06-24T14:59:55Z

An attempt at adding the better error message to Phobos: https://github.com/dlang/phobos/pull/6607

[dlangBugzillaToGithub]

github-bugzilla commented on 2018-11-19T07:45:08Z

Commits pushed to master at https://github.com/dlang/phobos

https://github.com/dlang/phobos/commit/887f7558e01535cc570b169d65358a7282361564
Fix Issue 13683 - More precise error message for wrong lambda

https://github.com/dlang/phobos/commit/f953e2c31137ce529da89a256ce9057cb06c8ec5
Merge pull request #6607 from wilzbach/fix-13683

[RFC] Fix Issue 13683 - More precise error message for wrong lambda

[dlangBugzillaToGithub]

b2.temp commented on 2018-11-19T08:10:02Z

More phobos algorithm that use lambda must be chnaged, in the same fashion, before closing. Seb's PR was "just" a RFC.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2023-03-11T15:15:51Z

static assert is an improvement over the constraint, but it still hides the actual error message which should be:

Error: use `!is` instead of `!=` when comparing with `null`

Removing the constraint allows the correct error to be seen. But then the constraint may be needed to disambiguate overloads. Without a compiler solution,  we could have a new trait - __traits(canPass, pred, range.front) which ignores the body of pred:

https://forum.dlang.org/post/lekofsbnstkoqpspramg@forum.dlang.org

In the absence of that, the best I've come up with is:

if (__traits(isTemplate, pred) || is(typeof(pred(range.front))))

Which doesn't check that pred can take that argument when pred is a lambda, but it doesn't hide the actual error unlike the constraint status quo or the static assert way.

A compiler solution might not work in the presence of overloads, because currently failing constraints are only shown when there are no overloads.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2023-03-11T15:19:05Z

> Without a compiler solution

BTW that would show the failing lambda with Phobos as is. The static assert PR discussion said a compiler solution is needed rather than having to update every template that accepts a lambda in Phobos.

[thewilsonator]

This now gives

onlineapp.d(5): Error: use `!is` instead of `!=` when comparing with `null`
/dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/searching.d(1622):        instantiated from here: `not!(Foo)`
/dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/searching.d(128):        instantiated from here: `find!(not, Foo[])`
onlineapp.d(5):        instantiated from here: `all!(Foo[])`