Confused about typing in ghci

Patrick Surry Patrick.Surry at
Wed Apr 2 16:54:36 EDT 2008

Thanks for the quick reply - I'm not sure I really understand exactly
what your rule means, but it's led me over to where it
seems possible that I might eventually figure it out :)

Any thoughts on the other question about where I can go to understand
Haskell's precedence/associativity rules better (and avoid so many


-----Original Message-----
From: Debian User [mailto:mdanish at] 
Sent: Wednesday, April 02, 2008 4:40 PM
To: Patrick Surry
Cc: glasgow-haskell-users at
Subject: Re: Confused about typing in ghci

On Wed, Apr 02, 2008 at 04:26:04PM -0400, Patrick Surry wrote:
> -- Why don't these (particularly g and g') all have the same type?
> Prelude> :t (\x -> x+1)
> (\x -> x+1) :: (Num a) => a -> a
> Prelude> let g = (\x -> x+1)
> Prelude> :t g
> g :: Integer -> Integer
> Prelude> let g' x = x + 1
> Prelude> :t g'
> g' :: (Num a) => a -> a

It's the monomorphism restriction in action, along with defaulting.
Basically, the rule is:

  Any binding of the form "g = ..." where there are no parameters
  *syntactically* must not be polymorphic with type-class

If you disable it with -fno-monomorphism-restriction then the
differences go away.  In addition, Haskell has some special-case
"defaulting" rules for turning Num classed type-variables into
concrete types, which explains the appearance of "Integer"

The reason for existence of this restriction is that such a "g" may be
recomputed for each use, much like a function call.  This could lead
to surprising behavior, though perfectly safe.  In Haskell, it's not
such a big deal, because of pure code, so some people prefer to turn
the restriction off.

Your options are to turn it off, always write syntactic parameters
like "g x = ...", or provide an explicit polymorphic type signature
like "g :: Num a => a -> a".

-- Matthew Danish -- user: mrd domain:
-- OpenPGP public key: C24B6010 on

DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

More information about the Glasgow-haskell-users mailing list