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
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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