[Haskell-cafe] Selda: confused about type signtures

[Marc Busqué]

Just for completeness about this solution:

On Mon, 23 Apr 2018, Li-yao Xia wrote:

> ```
> {-# LANGUAGE AllowAmbiguousTypes #-}
> {-# LANGUAGE ScopedTypeVariables #-}
>
> list :: forall s a. (Result (Cols s a), Columns (Cols s a))
>     => Table a -> IO [Res (Cols s a)]
> list table = withDB $ query (select table :: Query s _)

This gives error:

```
     • Couldn't match type ‘Res a’ with ‘Res (Cols s a)’
       Expected type: IO [Res (Cols s a)]
         Actual type: IO [Res a]
       NB: ‘Res’ is a type function, and may not be injective
     • In the expression: withDB $ query (select table :: Query s a)
       In an equation for ‘list’:
           list table = withDB $ query (select table :: Query s a)
     • Relevant bindings include
         table :: Table a (bound at src/Hedger/Backend.hs:30:6)
         list :: Table a -> IO [Res (Cols s a)]
           (bound at src/Hedger/Backend.hs:30:1)
    |
30 | list table = withDB $ query (select table :: Query s a)

  Couldn't match type ‘a’ with ‘Cols s a’
       ‘a’ is a rigid type variable bound by
         the type signature for:
           list :: forall s a.
                   (Result (Cols s a), Columns (Cols s a)) =>
                   Table a -> IO [Res (Cols s a)]
         at src/Hedger/Backend.hs:29:1-93
       Expected type: Query s a
         Actual type: Query s (Cols s a)
     • In the first argument of ‘query’, namely
         ‘(select table :: Query s a)’
       In the second argument of ‘($)’, namely
         ‘query (select table :: Query s a)’
       In the expression: withDB $ query (select table :: Query s a)
     • Relevant bindings include
         table :: Table a (bound at src/Hedger/Backend.hs:30:6)
         list :: Table a -> IO [Res (Cols s a)]
           (bound at src/Hedger/Backend.hs:30:1)
    |
30 | list table = withDB $ query (select table :: Query s a)
```

> -- or,                  ... (select @s table)
> -- with {-# LANGUAGE TypeApplications #-}
> ```

This indeed works!!

In either case, however, I need to add `{-# LANGUAGE FlexibleContexts #-}`

Now I have an interesting road in front of me in order to try to
understand it, along with Tom Ellis' isolated reproducing environment :)

Marc Busqué
http://waiting-for-dev.github.io/about/