#15147: Type checker plugin receives Wanteds that are not completely unflattened -------------------------------------+------------------------------------- Reporter: nfrisby | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.1 (Type checker) | Keywords: type checker | Operating System: Unknown/Multiple plugins | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- With the following, a plugin will receive a wanted constraint that includes a `fsk` flattening skolem. {{{ -- "Reduced" via the plugin type family F u v where {} type family G a b where {} data D p where DC :: (p ~ F x (G () ())) => D p }}} (Please ignore the apparent ambiguity regarding `x`; the goal is for the plugin to eliminate any ambiguity.) A do-nothing plugin that merely logs its inputs gives the following for the ambiguity check on DC. {{{ given [G] $d~_a4oh {0}:: (p_a4o2[sk:2] :: *) ~ (p_a4o2[sk:2] :: *) (CDictCan) [G] $d~~_a4oi {0}:: (p_a4o2[sk:2] :: *) ~~ (p_a4o2[sk:2] :: *) (CDictCan) [G] co_a4od {0}:: (G () () :: *) ~# (fsk_a4oc[fsk:0] :: *) (CFunEqCan) [G] co_a4of {0}:: (F x_a4o3[sk:2] fsk_a4oc[fsk:0] :: *) ~# (fsk_a4oe[fsk:0] :: *) (CFunEqCan) [G] co_a4og {1}:: (fsk_a4oe[fsk:0] :: *) ~# (p_a4o2[sk:2] :: *) (CTyEqCan) derived wanted [WD] hole{co_a4or} {3}:: (F x_a4o6[tau:2] fsk_a4oc[fsk:0] :: *) ~# (p_a4o2[sk:2] :: *) (CNonCanonical) untouchables [fsk_a4oe[fsk:0], fsk_a4oc[fsk:0], x_a4o3[sk:2], p_a4o2[sk:2]] }}} Note the `fsk_a4oc[fsk:0]` tyvar in the Wanted constraint, which is why I'm opening this ticket. Its presence contradicts the "The wanteds will be unflattened and zonked" claim from https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker#Callingpluginsfrom... section. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15147 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler