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From: duchier@cs.yale.edu (Denys Duchier)
Newsgroups: comp.object,comp.lang.misc,comp.lang.eiffel
Subject: Re: A Hard Problem for Static Type Systems
Message-ID: <DUCHIER.91Apr23205041@albania.cs.yale.edu>
Date: 24 Apr 91 00:50:41 GMT
References: <1991Apr20.010347.28984@leland.Stanford.EDU>
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In-reply-to: craig@leland.Stanford.EDU's message of 20 Apr 91 01:03:47 GMT


Haskell has the notion of classes, and below is the code taken
verbatim from the implementation of the Prelude. `instance (Ord a) =>
Ord [a] where ...' basically means, if `a' is a type of class Ord,
then `list of a' is also a type of class Ord, and the following
operations are defined on it...'.  Implementing this functionality
typically requires passing a dictionnary (as an additional argument)
that specifies the operations defined on type `a' as a member of class
Ord (something like this; I'm a little fuzzy on the exact details).

module PreludeListInst where

import PreludeRealCore

instance (Eq a) => Eq [a] where
  [] == [] = True
  (a:b) == (c:d) = a == c && b == d
  _ == _ = False

instance (Ord a) => Ord [a] where
  [] <= _ = True
  _ <= [] = False
  (a:b) <= (c:d) = a <= c || a == c && b <= d
  
  _ < [] = False
  [] < _ = True
  (a:b) < (c:d) = a < c || a == c && b < d

See "Report on the programming Language Haskell, A non-strict, Purely
Functional Language" (YALEU/DCS/RR-777).

Does this answer your question, or did I misunderstand the point you
were trying to make?

--Denys