Haskell: Parse error in pattern

Question

Who likes to tell me what is wrong with this code (syntactically)?

-- merge two sorted lists
mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX a:as b:bs res
    | a > b     = mergeX as b:bs a:res
    | otherwise = mergeX a:as bs b:res

Interpreter:

Parse error in pattern: mergeX

kennytm · Accepted Answer · 2011-12-19 13:06:52Z

40

You need some parenthesis:

mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX (a:as) (b:bs) res
    | a > b     = mergeX as (b:bs) (a:res)
    | otherwise = mergeX (a:as) bs (b:res)

The reason is because : has a lower precedence than function application, so mergeX a:as b:bs res will be parsed as:

(mergeX a):(as b):(bs res)

which is an error.

answered Dec 19, 2011 at 13:06

kennytm

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2 Comments

user905686 Over a year ago

Thanks, I remember that I had sth. like that before ;). Do you normally always use (x:xs) instead of x:xs to prevent mistakes?

kennytm Over a year ago

@user905686: In a pattern, yes.

Cubic · Accepted Answer · 2011-12-19 13:08:34Z

2

You need to put constructor patterns (or however they are called) in parantheses.

mergeX (a:as) (b:bs) res

answered Dec 19, 2011 at 13:08

Cubic

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