ghc-7.7.20130829: The GHC API

Safe HaskellNone




data OverlapFlag Source

The semantics allowed for overlapping instances for a particular instance. See Note [Safe Haskell isSafeOverlap] (in lhs) for a explanation of the isSafeOverlap field.



This instance must not overlap another


isSafeOverlap :: Bool

Silently ignore this instance if you find a more specific one that matches the constraint you are trying to resolve

Example: constraint (Foo [Int]) instances (Foo [Int]) (Foo [a]) OverlapOk Since the second instance has the OverlapOk flag, the first instance will be chosen (otherwise its ambiguous which to choose)


isSafeOverlap :: Bool

Like OverlapOk, but also ignore this instance if it doesn't match the constraint you are trying to resolve, but could match if the type variables in the constraint were instantiated

Example: constraint (Foo [b]) instances (Foo [Int]) Incoherent (Foo [a]) Without the Incoherent flag, we'd complain that instantiating b would change which instance was chosen


isSafeOverlap :: Bool

type InstEnv = UniqFM ClsInstEnvSource

lookupUniqueInstEnv :: (InstEnv, InstEnv) -> Class -> [Type] -> Either MsgDoc (ClsInst, [Type])Source

Look up an instance in the given instance environment. The given class application must match exactly one instance and the match may not contain any flexi type variables. If the lookup is unsuccessful, yield 'Left errorMessage'.

orphNamesOfClsInst :: ClsInst -> NameSetSource

Collects the names of concrete types and type constructors that make up the head of a class instance. For instance, given `class Foo a b`:

`instance Foo (Either (Maybe Int) a) Bool` would yield [Either, Maybe, Int, Bool]

Used in the implementation of :info in GHCi.