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December 05, 2017 cannot deduce template lambda from argument | ||||
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Hi, Having a little trouble understanding lambda type deduction. I have this lambda: immutable lambda(T) = (T n) => n * n; and if I call it with an explicit type it works else it errors with: lambda cannot deduce function from argument types !()(int) auto x = lambda!int(2); // ok auto x = lambda(2); // errors But if it's a normal template function then calling it without explicit type is ok. Thanks for any help! |
December 06, 2017 Re: cannot deduce template lambda from argument | ||||
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Posted in reply to aliak | On Tuesday, 5 December 2017 at 23:01:43 UTC, aliak wrote:
> immutable lambda(T) = (T n) => n * n;
Generally, you'd want to write
alias lambda = n => n * n;
instead. That said, I don't see any reason why your syntax shouldn't work, and would argue it's a bug. Please file it in Bugzilla.
--
Biotronic
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December 06, 2017 Re: cannot deduce template lambda from argument | ||||
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Posted in reply to Biotronic | On Wednesday, 6 December 2017 at 08:10:26 UTC, Biotronic wrote: > On Tuesday, 5 December 2017 at 23:01:43 UTC, aliak wrote: >> immutable lambda(T) = (T n) => n * n; > > Generally, you'd want to write > > alias lambda = n => n * n; > > instead. That said, I don't see any reason why your syntax shouldn't work, and would argue it's a bug. Please file it in Bugzilla. > > -- > Biotronic Ok thanks! Done: https://issues.dlang.org/show_bug.cgi?id=18037 Btw, in the case of your suggested approach, what if I want to constrain the parameter type? |
December 06, 2017 Re: cannot deduce template lambda from argument | ||||
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Posted in reply to aliak | On Wednesday, December 06, 2017 10:43:18 aliak via Digitalmars-d-learn wrote:
> On Wednesday, 6 December 2017 at 08:10:26 UTC, Biotronic wrote:
> > On Tuesday, 5 December 2017 at 23:01:43 UTC, aliak wrote:
> >> immutable lambda(T) = (T n) => n * n;
> >
> > Generally, you'd want to write
> >
> > alias lambda = n => n * n;
> >
> > instead. That said, I don't see any reason why your syntax shouldn't work, and would argue it's a bug. Please file it in Bugzilla.
> >
> > --
> >
> > Biotronic
>
> Ok thanks! Done: https://issues.dlang.org/show_bug.cgi?id=18037
>
> Btw, in the case of your suggested approach, what if I want to constrain the parameter type?
If you only want one type, then given n that type; I'm pretty sure that it would be
alias lambda = (int n) => n * n;
if you wanted an int. But if you want to do anything more complicated with it, it would make more sense to just turn it into a proper function template.
- Jonathan M Davis
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December 06, 2017 Re: cannot deduce template lambda from argument | ||||
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Posted in reply to Jonathan M Davis | On Wednesday, 6 December 2017 at 11:02:01 UTC, Jonathan M Davis wrote:
> If you only want one type, then given n that type; I'm pretty sure that it would be
>
> alias lambda = (int n) => n * n;
>
> if you wanted an int. But if you want to do anything more complicated with it, it would make more sense to just turn it into a proper function template.
>
> - Jonathan M Davis
Roight, I was more thinking along the lines of a little more complicated I guess :)
i.e.:
template lambda(T)
if (isIntegral!T)
{
alias lambda = (T n) => n * n;
}
But you're right, if it gets more complicated a proper function is probably better. Plus I just tried and it seems like you can't really constrain a variable template anyway. Unless there's some magic syntax I've overlooked in the docs.
Thanks!
Thanks!
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