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Today's Topics:
1. typeclass woes...how to constain a typeclass to be "closed"
under an operation.... (Nicholls, Mark)
----------------------------------------------------------------------
Message: 1
Date: Mon, 5 Jan 2015 10:54:32 +0000
From: "Nicholls, Mark" <[email protected]>
To: "[email protected]" <[email protected]>
Subject: [Haskell-beginners] typeclass woes...how to constain a
typeclass to be "closed" under an operation....
Message-ID:
<e7e7fdf32472ff48bb0e4d9dc4283d0e8384e...@mtvne-exmb02.mtvne.ad.viacom.com>
Content-Type: text/plain; charset="us-ascii"
> {-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts,
> FlexibleInstances, GADTs #-}
lets say I have a data type
> data S a = S a deriving Show
and a nice simple typeclass
> class Foo a where
> inc :: a -> S a
make Integer an instance and....
> instance Foo Integer where
> inc x = S x
brilliant...
now lets say I want my typeclass to enforce that any result of inc is also of a
type that is an instance of the typeclass
I'll follow my nose...which causes me a problem that makes me realise I maybe I
don't understand whats going on.
> class Bar a where
> inc' :: (Bar (S a)) => a -> S a
follow it to....
> instance Bar Integer where
> inc' x = S x
this WORKS!...even though "S Integer" is not an instance of Bar!...maybe I
don't understand.
> -- x = inc' (1 :: Integer)
This would go BOOM ...."no instance of (Bar (S Integer))"
...so that makes sense....
How do I force Haskell to check the constraint before I evaluate an expression?
follow my nose?
> -- class (Baz (S a)) => Baz a where
> -- inc'' :: a -> S a
BOOM ...."Cycle in class declaration"....
booo..but I suppose thats understandable....
so
a) is my use of typeclasses correct?...or am I missing something?
b) if so, how do I force the compiler to make sure my typeclass operations is
nicely closed (under the typeclass)?
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