CODEWITHSUNDEEP
haskellpattern-matchingoperator-precedenceparse-error
user905686
user905686

Reputation: 4634

Haskell: Parse error in pattern

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

Upvotes: 23

Views: 35684

Answers (2)

Cubic
Cubic

Reputation: 15683

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

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

Upvotes: 2

kennytm
kennytm

Reputation: 523344

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.

Upvotes: 40

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