Disambiguate IO actions in GHCi with :set +t

Currently GHCi's :set +t displays the same type for the following:

% ghci -ignore-dot-ghci
GHCi, version 7.8.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :set +t
Prelude> 'a'
'a'
it :: Char
Prelude> return 'a'
'a'
it :: Char
Prelude> return 'a' :: IO Char
'a'
it :: Char

A user at #haskell-beginners ran into a problem where they mistook an expression that defaulted to IO for a pure expression (Id . even <$> pure 5), :set +t was suggested but ended up showing the incorrect type:

ghci> Id . even <$> pure 5
Id False
it :: Id Bool

This is bad for pedagogical reasons, it would be an improvement to display the following:

Prelude> :set +t
Prelude> 'a'
'a'
it :: Char
Prelude> return 'a'
'a'
it :: Char <- return 'a' :: IO Char
Prelude> return 'a' :: IO Char
'a'
it :: Char <- return 'a' :: IO Char

mirroring existing syntax:

Prelude> :set -XScopedTypeVariables 
Prelude> a :: Char <- return 'a' :: IO Char
a :: Char

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	GHCi
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CC	hvr
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