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Today's Topics:
1. Re: Ambiguous type error: multiparam class + type alias
(Sylvain Henry)
----------------------------------------------------------------------
Message: 1
Date: Thu, 28 Sep 2017 12:58:16 +0200
From: Sylvain Henry <sylv...@haskus.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Ambiguous type error: multiparam
class + type alias
Message-ID: <2aab4bd9-f22c-e738-b467-1d9b7542f...@haskus.fr>
Content-Type: text/plain; charset=utf-8; format=flowed
On 28/09/2017 12:25, Baa wrote:
> Hello, Sylvain.
>
> Your solution assumes that I need to pass `@_ @AsTitle` anywhere where I
> call `repr`? So, instead of `repr n::AsTitle` or `repr n::Something AsTitle`
> (depends on implementation) I'll write `repr @_ @AsTitle n`, right?
Yes.
You could also avoid all this ambiguity/type application stuff with:
data AsTitle = AsTitle
class Repr a b where
repr :: a -> b -> Repr b
instance Repr Int AsTitle where
repr n _ = ...
...
print (repr n AsTitle)
(Repr b can be an associated type if you want to support different
representations)
>
>> You only add a constraint as follows:
>> repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b
> Yes... So, `b` is unbound/free type param. But I can bind it with func.
> deps, yes?
How? If you you bind it with a functional dependency on "a", you can
only have a single "b" for each "a".
Sylvain
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