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: [('b',[5,4]),('b',[1]),('b',[6])] 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. - Phil On Dec 6, 2007 9:39 AM, georg86 wrote:

Hello! 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',[5]),('b',[4]),('b',[1]),('b',[6])]: "Program error: pattern match failure: kgroup [('b',[5,4]),('b',[1]),('b',[6])]")

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 xs -- View this message in context: http://www.nabble.com/Pattern-matching-error-tf4957268.html#a14196413 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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