[Haskell-cafe] Simple type-class experiment turns out not so simple...

Steve Horne sh006d3592 at blueyonder.co.uk
Fri Jan 6 11:51:58 CET 2012

On 06/01/2012 10:29, Steffen Schuldenzucker wrote:
> On 01/06/2012 11:16 AM, Steve Horne wrote:
>> I was messing around with type-classes (familiarization exercises) when
>> I hit a probably newbie problem. Reducing it to the simplest case...
>> module BinTree ( WalkableBinTree, BT (Branch, Empty) ) where
>> -- n : node type
>> -- d : data item type wrapped in each node
>> class WalkableBinTree n where
>> wbtChildren :: n -> Maybe (n, n)
>> wbtData :: n -> Maybe d
> With 'd' not being mentioned anywhere, the signature of wbtData means 
> "forall d. n -> Maybe d". In particular, wbtData == const Nothing.
I'm not sure what to make of that. Even if the result of wbtData is 
always Nothing, surely it still has a static type?

>> I've tried varying a number of details. Adding another parameter to the
>> type-class (for the item-data type) requires an extension, and even then
>> the instance is rejected because (I think) the tree-node and item-data
>> types aren't independent.
> Did you try something like
> > {-# LANGUAGE MultiParamTypeClasses #-}
> > class WalkableBinTree n d where
> >   ... (same code as above, but 'd' is bound now)
> > ...
> > instance WalkableBinTree (BT x) x where
> >   ...
Precisely that. In that case, I get...

C:\_SVN\dev_trunk\haskell\examples>ghci -XMultiParamTypeClasses
GHCi, version 7.0.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> :load BinTree
[1 of 1] Compiling BinTree          ( BinTree.hs, interpreted )

     Illegal instance declaration for `WalkableBinTree (BT x) x'
       (All instance types must be of the form (T a1 ... an)
        where a1 ... an are *distinct type variables*,
        and each type variable appears at most once in the instance head.
        Use -XFlexibleInstances if you want to disable this.)
     In the instance declaration for `WalkableBinTree (BT x) x'
Failed, modules loaded: none.

If I specify both extensions (-XMultiParamTypeClasses and 
-XFlexibleInstances) it seems to work, but needing two language 
extensions is a pretty strong hint that I'm doing it the wrong way.

The goal is fairly obvious - to have type-classes for binary tree 
capabilities so that different implementations can support different 
subsets of those capabilities. Being able to walk a binary tree doesn't 
need ordering of keys, whereas searching does. A red-black tree needs 
somewhere to store it's colour in the node, yet the walking and 
searching functions don't need to know about that.

As far as I remember, none of the tutorials I've read have done this 
kind of thing - but it seemed an obvious thing to do. Obviously in the 
real world I should just use library containers, but this is about 
learning Haskell better in case a similar problem arises that isn't 
about binary trees.

How should I be handling this?

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