CODEWITHSUNDEEP
haskellmonads
Vamshi Surabhi
Vamshi Surabhi

Reputation: 537

How do you explain the associativity of the bind operator in this haskell expression?

Without any parentheses :

Prelude> [1,2] >>= \n -> ['a', 'b'] >>= \ch -> return (n, ch)
[(1,'a'),(1,'b'),(2,'a'),(2,'b')]

Parentheses assuming left associativity:

Prelude> ([1,2] >>= \n -> ['a', 'b']) >>= \ch -> return (n, ch)
<interactive>:22:49: Not in scope: `n'

Parentheses assuming right associativity:

Prelude> [1,2] >>= (\n -> ['a', 'b'] >>= \ch -> return (n, ch))
[(1,'a'),(1,'b'),(2,'a'),(2,'b')]

Isn't >>= left associative? When no parentheses are present, why does GHCi evaluate the expression as if >>= is right associative?

Upvotes: 17

Views: 1433

Answers (1)

kosmikus
kosmikus

Reputation: 19637

Yes, >>= is left associative. However, lambdas extend as far as possible. So the presence of the \n -> means that the only correct way to parse the expression is as

[1,2] >>= (\n -> ['a', 'b'] >>= \ch -> return (n, ch))

Note that your "left associativity" form

([1,2] >>= \n -> ['a', 'b']) >>= \ch -> return (n, ch)

isn't even scope-correct. The n in the final return is out of scope.

Upvotes: 19

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