6.11.2. Ambiguous types and the ambiguity check


Allow type signatures which appear that they would result in an unusable binding.

Each user-written type signature is subjected to an ambiguity check. The ambiguity check rejects functions that can never be called. For example:

f :: C a => Int

The idea is there can be no legal calls to f because every call will give rise to an ambiguous constraint. Indeed, the only purpose of the ambiguity check is to report functions that cannot possibly be called. We could soundly omit the ambiguity check on type signatures entirely, at the expense of delaying ambiguity errors to call sites. Indeed, the language extension AllowAmbiguousTypes switches off the ambiguity check.

Ambiguity can be subtle. Consider this example which uses functional dependencies:

class D a b | a -> b where ..
h :: D Int b => Int

The Int may well fix b at the call site, so that signature should not be rejected. Moreover, the dependencies might be hidden. Consider

class X a b where ...
class D a b | a -> b where ...
instance D a b => X [a] b where...
h :: X a b => a -> a

Here h’s type looks ambiguous in b, but here’s a legal call:

...(h [True])...

That gives rise to a (X [Bool] beta) constraint, and using the instance means we need (D Bool beta) and that fixes beta via D’s fundep!

Behind all these special cases there is a simple guiding principle. Consider

f :: type
f = ...blah...

g :: type
g = f

You would think that the definition of g would surely typecheck! After all f has exactly the same type, and g=f. But in fact f’s type is instantiated and the instantiated constraints are solved against the constraints bound by g’s signature. So, in the case an ambiguous type, solving will fail. For example, consider the earlier definition f :: C a => Int:

f :: C a => Int
f = ...blah...

g :: C a => Int
g = f

In g’s definition, we’ll instantiate to (C alpha) and try to deduce (C alpha) from (C a), and fail.

So in fact we use this as our definition of ambiguity: a type ty is ambiguous if and only if ((undefined :: ty) :: ty) would fail to typecheck. We use a very similar test for inferred types, to ensure that they too are unambiguous.

Switching off the ambiguity check. Even if a function has an ambiguous type according to the “guiding principle”, it is possible that the function is callable. For example:

class D a b where ...
instance D Bool b where ...

strange :: D a b => a -> a
strange = ...blah...

foo = strange True

Here strange’s type is ambiguous, but the call in foo is OK because it gives rise to a constraint (D Bool beta), which is soluble by the (D Bool b) instance.

Another way of getting rid of the ambiguity at the call site is to use the TypeApplications extension to specify the types. For example:

class D a b where
  h :: b
instance D Int Int where ...

main = print (h @Int @Int)

Here a is ambiguous in the definition of D but later specified to be Int using type applications.

AllowAmbiguousTypes allows you to switch off the ambiguity check. However, even with ambiguity checking switched off, GHC will complain about a function that can never be called, such as this one:

f :: (Int ~ Bool) => a -> a

Sometimes AllowAmbiguousTypes does not mix well with RankNTypes. For example:

foo :: forall r. (forall i. (KnownNat i) => r) -> r
foo f = f @1

boo :: forall j. (KnownNat j) => Int
boo = ....

h :: Int
h = foo boo

This program will be rejected as ambiguous because GHC will not unify the type variables j and i.

Unlike the previous examples, it is not currently possible to resolve the ambiguity manually by using TypeApplications.


A historical note. GHC used to impose some more restrictive and less principled conditions on type signatures. For type forall tv1..tvn (c1, ...,cn) => type GHC used to require

  1. that each universally quantified type variable tvi must be “reachable” from type, and
  2. that every constraint ci mentions at least one of the universally quantified type variables tvi. These ad-hoc restrictions are completely subsumed by the new ambiguity check.