[Haskell-cafe] MonadFail with type parameter for error message

[Daneel Yaitskov]

On Tue, Nov 30, 2021 at 2:20 PM Brandon Allbery <allbery.b at gmail.com> wrote:

> Because the primary purpose of fail is to provide for failed pattern
> matches, which invoke it with a String representing the pattern match
> error.
>

I see the purpose, but fail with just literal string is not very
informative.
It is easier for debugging supplying an error message with Showable
parameters.
Custom Prelude has show :: a - >Text so fail looks like:

    oops -> fail $ "functionX is not implemented for " <> show oops



> On Tue, Nov 30, 2021 at 2:16 PM Daneel Yaitskov <dyaitskov at gmail.com>
> wrote:
> >
> > Dear Cafe,
> >
> > MonadFail.fail takes String.
> > I wasn't able to find MonadFail for custom error type.
> > Is there any proposals to base?
> >
> > Let's say Data.Text, which gains popularity with OverloadedStrings
> extensions.
> >
> > class MonadFail m where
> >   fail :: String -> m a
> >
> > Why not ?
> > class MonadFail m where
> >   fail :: (forall s. IsString s => s) -> m a
> >
> > class MonadFailWith m s where
> >   fail :: s -> m a
> >
> >
> >
> >
> >
> >
> >
> > --
> >
> > Best regards,
> > Daniil Iaitskov
> >
> >
> >
> > _______________________________________________
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>
>
>
> --
> brandon s allbery kf8nh
> allbery.b at gmail.com
>


-- 

Best regards,
Daniil Iaitskov
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