#10166: GHCi: internal error: stg_ap_p_ret -------------------------------------+------------------------------------- Reporter: andreas.abel | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: GHCi | Version: 7.8.4 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by monoidal): * status: new => closed * resolution: => invalid Comment: Letting X be a shortcut for `Const (Suc (Suc Zero))`, your program makes an evaluation step {{{ eval (App Pred X) => coerce $ evalP (App Pred X)` }}} and attempts to print this as a `Nat`, but `evalP (App Pred X)` is a function (evalP takes two arguments), that's an invalid coercion. If you change your `eval` to {{{#!hs eval :: Exp -> a eval (Const n) = coerce n eval Pred = coerce pr eval (App f e) = coerce (eval f (eval e)) pr Zero = Zero pr (Succ n) = n }}} then it works on my machine, but you should use `Any` for such tricks (https://hackage.haskell.org/package/ghc-prim-0.3.1.0/docs/GHC- Prim.html#v:unsafeCoerce35-). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10166#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler