On Fri, 20 Apr 2018, Tom Ellis wrote:
Did you try applying the suggested fix? Explicitly, add
{-# LANGUAGE FlexibleContexts #-}
at the top of your file to permit this.
It doesn't work neither. In this case the error is: ``` • Couldn't match type ‘selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)’ with ‘selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s a)’ Expected type: Table a -> IO [selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s a)] Actual type: Table a -> IO [selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)] NB: ‘selda-0.1.12.1:Database.Selda.Compile.Res’ is a type function, and may not be injective The type variable ‘s0’ is ambiguous • In the ambiguity check for the inferred type for ‘list’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the inferred type list :: forall s a. (selda-0.1.12.1:Database.Selda.Column.Columns (selda-0.1.12.1:Database.Selda.Column.Cols s a), selda-0.1.12.1:Database.Selda.Compile.Result (selda-0.1.12.1:Database.Selda.Column.Cols s a)) => Table a -> IO [selda-0.1.12.1:Database.Selda.Compile.Res (selda-0.1.12.1:Database.Selda.Column.Cols s a)] | 36 | list table = withDB $ query (select table) ``` Anyway, I'm more interested in understanding an explicit type signature, in this case. Marc Busqué http://waiting-for-dev.github.io/about/