Re: [Haskell-cafe] inference with implicitparams seems broken

24 Mar 2022


      Indeed, with the monomorphism restriction disabled it works without 
issues. is this by design/expected though? Morally I feel it is already 
monomorph, it's just missing an argument which needs to be provided. 
Would there be interest in fixing this in GHC?

On 3/23/22 15:56, J. Reinders wrote:
...
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  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-.... 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
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