[Haskell-cafe] Pattern matching error
philip.weaver at gmail.com
Thu Dec 6 13:03:02 EST 2007
If you add a third pattern, you can see exactly what it's failing to match:
kmerge x = error (show x)
In order to do this, you just need to add Show constraints for a and b in
the type of kmerge:
kmerge :: (Show a, Show b, Eq a) => [(a,[b])]->[(a,[b])]
You'll find that the pattern that it's failing to match is:
Because you combined 5 and 4 and passed that on to your recursive 'kmerge'
call. Your patterns only cover the case where there is exactly one element
in the list.
On Dec 6, 2007 9:39 AM, georg86 <georg_straube at web.de> wrote:
> I need to write a function which should is supposed to merge multiple
> entries with the same
> key (out of a sorted key-value-list) into one single entry.
> However, I keep getting a pattern matching error.
> (For example, for input [('b',),('b',),('b',),('b',)]:
> "Program error: pattern match failure: kgroup
> Thank you very much for your help.
> kmerge::Eq a => [(a,[b])]->[(a,[b])]
> kmerge  = 
> kmerge ((x,[y]):) = [(x,[y])]
> kmerge ((x,[y]):(u,[v]):xs) | x == u = kmerge ((x,[y,v]):xs)
> | otherwise = (x,[y]):(u,[v]):kmerge
> View this message in context:
> Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
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