GHC infers over-general types

Consider

f x = x + head

GHC will (with no flags) will compile this almost-certainly-wrong program, giving f the type

    f :: forall a. Num ([a] -> a) => ([a] -> a) -> [a] -> a

There's nothing wrong with that type, and in principle someone might later define a suitable instance of Num, but (a) it's not Haskell 98, and (b) it's unlikely to be right, so we've deferred a type error from the definition site of f to the (perhaps much later) call site.

I think GHC should complain, and require a type signature if that's what you really want. This came up on Stackoverflow

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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