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...
- TypeApplications: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts...
- AllowAmbiguousTypes: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts...
Li-yao
Thanks Li-yao. I'll studiy it thoroughly. It looks promising. Marc Busqué http://waiting-for-dev.github.io/about/