[Haskell-cafe] Re: surprised by type class binding -- is this a bug?

[Greg Meredith]

Haskellians,

Belay that. i see the problem.

Best wishes,

--greg

On Dec 6, 2007 11:11 PM, Greg Meredith <lgreg.meredith at biosimilarity.com>
wrote:

> Haskellians,
>
> i'm sure i don't understand type classes, yet. Still, i was surprised at
> ghci's response to the code below. Clues gratefully accepted.
>
> Best wishes,
>
> --greg
>
> -- transcript
> -- Prelude> :l grn
> -- [1 of 1] Compiling GeneticRegulatoryNetwork ( grn.hs, interpreted )
>
> -- grn.hs:33:35:
> --    Couldn't match expected type `b1' (a rigid variable)
> --       against inferred type `b' (a rigid variable)
> --      `b1' is bound by the type signature for `sequence' at grn.hs:25:36
> --      `b' is bound by the instance declaration at grn.hs:31:0
> --      Expected type: [b1]
> --      Inferred type: [b]
> --    In the expression: molecules
> --    In the definition of `sequence':
> --    sequence (Site l1 molecules) = molecules
> -- Failed, modules loaded: none.
> -- Prelude>
>
> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
> -- -*- mode: Haskell;-*-
> -- Filename:    grn.hs
> -- Authors:     lgm
> -- Creation:    Thu Dec  6 15:38:26 2007
> -- Copyright:   Not supplied
> -- Description:
> -- ----------------------------------------------------------------
>
> module GeneticRegulatoryNetwork
>     where
>
> data Segment p =
>     Nil
>   | Section [p] (Segment p)
>     deriving (Eq, Show)
>
> class BindingMolecule b l | b -> l where
>     name        :: b -> l
>     complement  :: b -> b
>     complements :: b -> Bool
>
> class Locale s l1 l2 | s -> l1 l2 where
>     label       :: s -> l1
>     sequence    :: (BindingMolecule b l2) => s -> [b]
>     provides    :: (BindingMolecule b l2) => s -> [b] -> Bool
>     matches     :: (BindingMolecule b l2) => s -> [b] -> Bool
>
> data (BindingMolecule b l2) => Site b l1 l2 = Site l1 [b] deriving (Eq,
> Show)
>
> instance (BindingMolecule b l2) => Locale (Site b l1 l2) l1 l2 where
>     label    (Site l1 _)  = l1
>     sequence (Site l1 molecules) = molecules
>     provides (Site l1 molecules) molecules' = False -- tbd
>     matches  (Site l1 molecules) molecules' = False -- tbd
>
>
>
> --
> L.G. Meredith
> Managing Partner
> Biosimilarity LLC
> 505 N 72nd St
> Seattle, WA 98103
>
> +1 206.650.3740
>
> http://biosimilarity.blogspot.com




-- 
L.G. Meredith
Managing Partner
Biosimilarity LLC
505 N 72nd St
Seattle, WA 98103

+1 206.650.3740

http://biosimilarity.blogspot.com
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