[Haskell-cafe] Pattern matching error
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.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern matching error
kmerge ((x,[y]):[]) = [(x,[y])] Matching on [y] like this will only match lists with a single element (and bind y to that element). You probably just want to say kmerge ((x,y):[]) = [(x,y)] etc., which will bind y to the entire list. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern matching error
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 [EMAIL PROTECTED] 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.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern matching error
Philip Weaver [EMAIL PROTECTED] writes:
You'll find that the pattern that it's failing to match is:
[('b',[5,4]),('b',[1]),('b',[6])]
You could also use ghc with -Wall, which will tell you exactly which
cases you've omitted.
-k
--
If I haven't seen further, it is by standing in the footprints of giants
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
