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Here's my code (fun.hs):

{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}

import Control.Monad.Reader

instance Eq (a -> b) where
    _ == _ = error "whoops"

instance Show (a -> b) where
    show = const "<fun>"

instance (Num a, Monad m, Eq (m a), Show (m a)) => Num (m a) where
    fromInteger = return . fromInteger
    abs = liftM abs
    signum = liftM signum
    (+) = liftM2 (+)
    (*) = liftM2 (*)
    (-) = liftM2 (-)

instance (Fractional a, Monad m, Num (m a)) => Fractional (m a) where
    fromRational = return . fromRational
    (/) = liftM2 (/)

instance (Floating a, Monad m, Fractional (m a)) => Floating (m a) where
    pi = return pi
    exp = liftM exp
    log = liftM log
    sin = liftM sin
    cos = liftM cos
    tan = liftM tan
    sinh = liftM sinh
    cosh = liftM cosh
    tanh = liftM tanh
    asin = liftM sin
    acos = liftM cos
    atan = liftM tan
    asinh = liftM sinh
    acosh = liftM cosh
    atanh = liftM tanh

Test session:

$ ghci fun.hs
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.6, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( fun.hs, interpreted )
Ok, modules loaded: Main.
*Main> 1 2

<interactive>:1:0:
    No instance for (Num (t1 -> t))
      arising from the literal `1' at <interactive>:1:0-2
    Possible fix: add an instance declaration for (Num (t1 -> t))
    In the expression: 1 2
    In the definition of `it': it = 1 2

Expected result:

Loading package mtl-1.0 ... linking ... done.
1

This used to work in ghci-6.4. What's also weird is that manually lifting one number seems to help:

*Main> (1 + 2) undefined

<interactive>:1:1:
    No instance for (Num (a -> t))
      arising from the literal `1' at <interactive>:1:1
    Possible fix: add an instance declaration for (Num (a -> t))
    In the first argument of `(+)', namely `1'
    In the expression: (1 + 2) undefined
    In the definition of `it': it = (1 + 2) undefined
*Main> (1 + const 2) undefined
3

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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