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/