[Haskell-cafe] inference with implicitparams seems broken

[J. Reinders]

It’s due to the monomorphism restriction [1]. If you give `baz` an argument it does infer the type correctly:

```
foo :: Int
foo = bar 
  where 
    bar = withX 42 (baz ())
    baz () = somethingThanNeedsX
```

Or you can disable the monomorphism restriction:

```
{-# LANGUAGE NoMonomorphismRestriction #-}
```

[1] https://wiki.haskell.org/Monomorphism_restriction

> On 23 Mar 2022, at 15:48, rowan goemans <goemansrowan at gmail.com> wrote:
> 
> Hello everyone,
> 
> I wonder why the following taken from an stackoverflow answer doesn't work: https://stackoverflow.com/questions/71184922/is-it-possible-to-infer-a-type-that-is-a-reflection-like-closure. Code of the answer posted here verbatim:
> 
> ```
> {-# LANGUAGE ImplicitParams #-}
> {-# LANGUAGE RankNTypes #-}
> 
> module Temp where
> 
> withX :: Int -> ((?x :: Int) => r) -> r
> withX x r =
>   let ?x = x
>   in r
> 
> somethingThanNeedsX :: (?x :: Int) => Int
> somethingThanNeedsX = ?x + 2
> 
> foo :: Int
> foo = bar
>   where
>     bar = withX 42 baz
> 
>     baz = somethingThanNeedsX
> ```
> 
> This won't compile with the error message:
> 
> ```
> Orig.hs:19:11: error:
>     • Unbound implicit parameter (?x::Int)
>         arising from a use of ‘somethingThanNeedsX’
>     • In the expression: somethingThanNeedsX
>       In an equation for ‘baz’: baz = somethingThanNeedsX
>       In an equation for ‘foo’:
>           foo
>             = bar
>             where
>                 bar = withX 42 baz
>                 baz = somethingThanNeedsX
>    |
> 19 |     baz = somethingThanNeedsX
> ```
> 
> If you give `baz` a type signature which includes the implicit param it compiles. But I think this is a bug. GHC should not require a type signature here. I tried searching for a GHC ticket but I couldn't find one. But that might also be down to me not using the right query.
> 
> Anyway my question is, is this expected behavior? This really makes implicit params less ergonomic.
> 
> Best regards,
> 
> Rowan Goemans
> 
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.