Cannot assign in haskell prompt

Question

At the GHCi prompt,

I am able to do,

Prelude Text.Parsec> :t (oneOf "aeiou")

and get (oneOf "aeiou") :: Stream s m Char => ParsecT s u m Char

But when I do,

let s = oneOf "aeiou"

I get the following error,

interactive>:36:9:
    No instance for (Stream s0 m0 Char)
      arising from a use of `oneOf'
    Possible fix: add an instance declaration for (Stream s0 m0 Char)
    In the expression: oneOf "aeiou"
    In an equation for `m': m = oneOf "aeiou"

Could someone please tell me why I am not able to assign to s?

Maybe because of type defaulting. What about when you do `let s = (oneOf "aeiou") :: Stream s m Char => ParsecT s u m Char`? — Satvik, Aug 17 '13 at 11:47
possible duplicate of [What is the monomorphism restriction?](http://stackoverflow.com/questions/32496864/what-is-the-monomorphism-restriction) — Bakuriu, Sep 11 '15 at 09:08

score 6 · Accepted Answer · edited May 23 '17 at 10:26

6

This is the monomorphism restriction in action.

This question is closely related, though it isn't about the ghci let but the idea is the same.

edited May 23 '17 at 10:26

Community

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answered Aug 17 '13 at 14:06

n. m. could be an AI

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Cannot assign in haskell prompt

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