#14170: 8.2.1 regression: GHC fails to simplify `natVal` -------------------------------------+------------------------------------- Reporter: vagarenko | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.2 Component: Compiler | Version: 8.2.1 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 RyanGlScott): I was able to track this down to commit 1fcede43d2b30f33b7505e25eb6b1f321be0407f (`Introduce GHC.TypeNats module, change KnownNat evidence to be Natural`), which hints at the problem. In that commit, we switched the internal representation of `Nat`s from `Integer`s to `Natural`s (from `Numeric.Natural`). For whatever reason, however, `Natural` values don't seem to simplify as well as `Integers`, as evidenced by this simpler program: {{{#!hs module Bug where import Numeric.Natural foo :: Natural foo = 0 }}} which also produces essentially identical core: {{{#!hs -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0} Bug.foo1 :: Integer [GblId, Caf=NoCafRefs, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 100 0}] Bug.foo1 = 0 -- RHS size: {terms: 39, types: 12, coercions: 0, joins: 0/0} foo :: Natural [GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False, WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 126 60}] foo = case Bug.foo1 of { integer-gmp-1.0.1.0:GHC.Integer.Type.S# i#_a2bZ -> case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.>=# i#_a2bZ 0#) of { False -> GHC.Natural.underflowError @ Natural; True -> GHC.Natural.NatS# (GHC.Prim.int2Word# i#_a2bZ) }; integer-gmp-1.0.1.0:GHC.Integer.Type.Jp# dt_a2c9 -> case GHC.Prim.uncheckedIShiftRL# (GHC.Prim.sizeofByteArray# dt_a2c9) 3# of { __DEFAULT -> case GHC.Prim.sizeofByteArray# dt_a2c9 of { __DEFAULT -> GHC.Natural.NatJ# dt_a2c9; 0# -> GHC.Natural.underflowError @ Natural }; 1# -> case GHC.Prim.indexWordArray# dt_a2c9 0# of wild2_a2cd { __DEFAULT -> GHC.Natural.NatS# wild2_a2cd } }; integer-gmp-1.0.1.0:GHC.Integer.Type.Jn# ipv_a2cg -> GHC.Natural.underflowError @ Natural } }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14170#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler