CODEWITHSUNDEEP
haskell
George
George

Reputation: 7327

Why are parentheses needed with `:` in this context?

Typing the following into a GHC interpreter yields no error:

let head' (x:_) = x

But if we remove the parentheses:

let head' x:_ = x

...we obtain:

Parse error in pattern: head'

Why are the parentheses necessary?

Upvotes: 2

Views: 126

Answers (1)

comingstorm
comingstorm

Reputation: 26117

In Haskell, function application has higher precedence than any operator, and pattern-matching reflects that.

So, without the parentheses, head' x:_ is parsed as (head' x):_, which doesn't make sense in this context, and causes an error.

Upvotes: 10

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