#14969: Underconstrained typed holes are non-performant -------------------------------------+------------------------------------- Reporter: johnleo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: TypedHoles Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Compile-time | Test Case: performance bug | ghci/scripts/T14969 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * failure: None/Unknown => Compile-time performance bug Comment: Note that it doesn't loop infinitely, but rather takes a very long time to compute: {{{ $ time ghc/inplace/bin/ghc-stage2 -XFlexibleContexts -e "3 _ 4" <interactive>:0:1: error: • Could not deduce (Num t1) from the context: (Num t, Num (t2 -> t -> t3)) bound by the inferred type for ‘it’: forall t t2 t3. (Num t, Num (t2 -> t -> t3)) => t3 at <interactive>:0:1-5 The type variable ‘t1’ is ambiguous • In the ambiguity check for the inferred type for ‘it’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the inferred type it :: forall t1 t2 t3. (Num t1, Num (t2 -> t1 -> t3)) => t3 real 0m32.219s user 0m32.176s sys 0m0.084s }}} In other words, this is the same problem observed in https://ghc.haskell.org/trac/ghc/ticket/10946#comment:9. This is good home for this particular bug. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14969#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler