Language.Haskell.Syntax.Decls.Overlap

Constructors

• NoOverlap (XOverlapMode pass)

  This instance must not overlap another NoOverlap instance. However, it may be overlapped by Overlapping instances, and it may overlap Overlappable instances.

• Overlappable (XOverlapMode pass)

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

  Example: constraint (Foo [Int]) instance Foo [Int] instance {-# OVERLAPPABLE #-} Foo [a]

  Since the second instance has the Overlappable flag, the first instance will be chosen (otherwise its ambiguous which to choose)

• Overlapping (XOverlapMode pass)

  Silently ignore any more general instances that may be used to solve the constraint.

  Example: constraint (Foo [Int]) instance {-# OVERLAPPING #-} Foo [Int] instance Foo [a]

  Since the first instance has the Overlapping flag, the second---more general---instance will be ignored (otherwise it is ambiguous which to choose)

• Overlaps (XOverlapMode pass)

  Equivalent to having both Overlapping and Overlappable flags.

• Incoherent (XOverlapMode pass)

  Behave like Overlappable and Overlapping, and in addition pick an arbitrary one if there are multiple matching candidates, and don't worry about later instantiation

  Example: constraint (Foo [b]) instance {-# INCOHERENT -} Foo [Int] instance Foo [a] Without the Incoherent flag, we'd complain that instantiating b would change which instance was chosen. See also Note [Incoherent instances] in GHC.Core.InstEnv

• NonCanonical (XOverlapMode pass)

  Behave like Incoherent, but the instance choice is observable by the program behaviour. See Note [Coherence and specialisation: overview].

  We don't have surface syntax for the distinction between Incoherent and NonCanonical instances; instead, the flag `-f{no-}specialise-incoherents` (on by default) controls whether INCOHERENT instances are regarded as Incoherent or NonCanonical.

• XOverlapMode !(XXOverlapMode pass)

  The Trees That Grow extension point constructor.