[Haskell-cafe] Selda: confused about type signtures

[Marc Busqué]

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

> Hi Marc,
>
>> ```
>> list :: (Result (Cols s a), Columns (Cols s a))  => Table a -> IO [Res 
>> (Cols s a)]
>> list table = withDB $ query (select table)
>> ```
>
> The only occurence of `s`/`s0` that is not ambiguous is between `select` and 
> `query`, as the first argument of the data type `Query`. Everywhere else, 
> there is a type family in the way which prevents unification; for example 
> `Cols s a ~ Cols s0 a` does not imply `s ~ s0`.
>
> You can use a type annotation or TypeApplications to instantiate the `s0` 
> between `query` and `select`. This requires ScopedTypeVariables, and an 
> explicit `forall` at the top to make the type variables available in the 
> definition body.
>
> Note that the type of `list` is also ambiguous for the aforementioned 
> reasons, since `s` is only an argument of the type family `Res` (and `Cols`). 
> You will have to AllowAmbiguousTypes to define it and write `list @s` (with 
> TypeApplications) to use it.
>
> ```
> {-# 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 _)
> -- or,                  ... (select @s table)
> -- with {-# LANGUAGE TypeApplications #-}
> ```
>
> References in the GHC manual:
>
> - ScopedTypeVariables: 
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#lexically-scoped-type-variables
>
> - TypeApplications: 
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#visible-type-application
>
> - AllowAmbiguousTypes: 
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#ambiguous-types-and-the-ambiguity-check
>
> Li-yao

Thanks Li-yao. I'll studiy it thoroughly. It looks promising.

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