GHC.Hs.Decls.Overlap

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

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)

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)

Equivalent to having both Overlapping and Overlappable flags.

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

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.

The Trees That Grow extension point constructor.