== 3.1 Implication Constraints == During type inference, GHC uses ''implication constraints'', which do not appear in Haskell source code directly. An implication constraint is, roughly, of the form `G ⇒ W`, where `W` is a collection of constraints

#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): http://yav.github.io/publications/improving-smt-types.pdf that need to be discharged, and `G` are assumptions that may be used while discarding `W`. In the GHC source code, the constraints in `G` are referred to as ''given constraints'', while the ones in `W` are known as wanted constraints. The intuition behind an implication constraint is that `W` contains the constraints that were collected while checking a program fragment, while `G` contains local assumptions that are available only in this particular piece of code. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5927#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler