[Haskell-cafe] Selda: confused about type signtures

[Tom Ellis]

On Fri, Apr 20, 2018 at 08:33:10PM +0200, Marc Busqué wrote:
> Not adding a type signature to `list` produces following compilation
> error:
> 
[...]
>   (Use FlexibleContexts to permit this)
>
> Why is that it works with concrete types but not with a type variable
> that takes the same concrete type in its both placeholders?

Did you try applying the suggested fix?  Explicitly, add

{-# LANGUAGE FlexibleContexts #-}

at the top of your file to permit this.