[Haskell-cafe] Problem w/YAHT code example

michael rice nowgate at yahoo.com
Fri May 29 00:01:26 EDT 2009

Thanks! I just copied the code from the PDF file. Whatever that character was it wasn't the correct one.


--- On Thu, 5/28/09, Andrew Wagner <wagner.andrew at gmail.com> wrote:

From: Andrew Wagner <wagner.andrew at gmail.com>
Subject: Re: [Haskell-cafe] Problem w/YAHT code example
To: "michael rice" <nowgate at yahoo.com>
Cc: haskell-cafe at haskell.org
Date: Thursday, May 28, 2009, 10:01 PM

I'm not sure what you're using at the end of the identifier "search'", but it needs to be the single quote that's on the same key as the double quotes. I suspect that's where it's blowing up.

On Thu, May 28, 2009 at 9:56 PM, michael rice <nowgate at yahoo.com> wrote:

This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below.



[michael at localhost ~]$ ghci

GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help

Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.

Loading package base ... linking ... done.
Prelude> :l graph

[1 of 1] Compiling Main             ( graph.hs, interpreted )

graph.hs:6:24: lexical error at character '\8217'
Failed, modules loaded: none.


Prelude> data Graph v e = Graph [(Int,v)] [(Int,Int,e)]

search :: Graph v e -> Int -> Int -> Maybe [Int]
search g@(Graph vl el) src dst

    | src == dst = Just [src]
    | otherwise = search’ el

    where search’ [] = Nothing
          search’ ((u,v,_):es)

              | src == u =
                case search g v dst of

                  Just p -> Just (u:p)
                  Nothing -> search’ es

              | otherwise = search’ es


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