#10688: Incorrect program produced, different result with equivalent code -------------------------------------+------------------------------------- Reporter: Ideophage | Owner: Type: bug | Status: closed Priority: lowest | Milestone: Component: Compiler | Version: 7.10.1 Resolution: invalid | Keywords: Operating System: Linux | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Ideophage): Replying to [comment:6 simonpj]:
Interesting. Now that you understand what went wrong, you could write a note explaining the trap you fell into, and share it with others. It is, after all, rather surprising when you are told "omit a type signature and you get an infinite loop".
Simon
I don't know if it's always the case, but according to my tests, the type inferrence algorithm specialize the types without class constraints (however, if we enter `:t (unlines . map show)` in GHCi, the polymorphic type is inferred). For instance, the following code either prints "1" or generates an error depending on whether we comment out or not the definition of `print_b` (which is not used) : {{{#!hs a = (2^64) `div` (2^64) -- 64 or 32 b = length [] show' = show print_a = print $ show' a -- print_b = print $ show' b main = print_a }}} This is because if we apply `show'` to `b`, then the inferred type of `show'` becomes `Int → String` (I expected it to be the same as `show`) and because we also apply it to `a`, the inferred type of `a` becomes `Int`. The « types implication graph » of the code I first gave was more complex, but there is nothing more than this involved. I thought it was a bug because I assumed GHC's Haskell was referentially transparent even without explicit types annotations (so that expanding a definition, adding a definition, etc. was not supposed to make a program do something else), but I don't know what is feasible. The infinite loop is due to the fact that the termination rely on arithmetic properties of `Integer`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10688#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler