Re: Narrowing vs. rewriting

From: Philip Wadler <wadler_at_research.bell-labs.com>
Date: Thu, 03 Jul 1997 12:52:04 -0400

I don't find either Sergio's and Harold's examples very convincing,
as both are easy to program in Haskell. I'm sure there are examples
of programs that are easy to write in Curry but not so easy to write in
Haskell, let's see some please! Cheers, -- P


1. Sergio's example:

        permute [] = []
        permute (x:xs) | u++v == permute xs = u++[x]++v

The same in Haskell (returns a list of all possible permutations):

        permute [] = [[]]
        permute (x:xs) = [ take i ys ++ [x] ++ drop i ys
                         | ys <- permute xs, i <- [0..length ys] ]

This is slightly longer, but makes explicit the order in which the
permutations are examined. The functions (++), take, drop, and length
are part of the standard library.


2. Harold's example:

        pairlists([],[]) = []
        pairlists([X|L],[Y|M]) = [[X,Y]|pairlists(L,M)]

        numbered([],N).
        numbered([[X,N]|R],N) :- numbered(R,+(N,1)).

        before([X1,Y1],[X2,Y2]) :- string<(X1,X2).

        serialise(L) = R if numbered(qsort[before](pairlists(L,R)),1)

The same in Haskell:

        before (x1,y1) (x2,y2) = x1 <= x2
        serialise l = map fst (sortBy before (zip (l, [1..])))
                        
The functions (<=), map, fst, sortBy, and zip are part of the standard
library.


The library functions can be found in:
  http://haskell.systemsz.cs.yale.edu/onlinereport/standard-prelude.html
  http://haskell.systemsz.cs.yale.edu/onlinelibrary/list.html
And here's the Haskell home page:
  http://www.haskell.org/
Received on Thu Jul 03 1997 - 19:54:00 CEST

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