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