#15421: Refactor (~) to reduce the superclass stack -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently (see `Note [The equality types story]` in `TysPrim`) we have {{{ -- Hetereogeneous equality class a ~# b => a ~~ b instance a ~# b => a ~~ b -- Homogeneous equality class a ~~ b => (a :: k) ~ (b :: k) instance a ~~ b => a ~ b }}} Note that `(~#)` is a superclass of `(~~)`, and `(~~)` is a superclass of `(~)`. This means that in the common case of using `(~)` we need two superclass selections to get to the `(~#)` we want. Nothing really wrong with that, but it bloats programs, and is confusing to read when debugging. I propose to change this to {{{ -- Homogeneous equality class a ~# b => (a :: k) ~ (b :: k) instance a ~# b => a ~ b }}} That is, implement `(~)` in precisely the same way as `(~~)`. This makes `(~)` a tiny bit more baked-in to the compiler, but in exchange it behaves in the same way as `(~~)`, instead of behaving in a slightly different way. There should be no observable effect for users. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15421 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler