Basic Fibonacci function using Word causes ghci to panic. - 6.10.1

[axman6]

When inputting the function:

let fib :: Word -> Word; fib 0 = 1; fib 1 = 1; fib n = l + r where l = fib (n-2); r = fib (n-1)

GHCi produces a panic error:

ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-apple-darwin):
	schemeE(AnnCase).my_discr __word 0

It has been confirmed on both OS X 10.5.5 and linux

[axman6]

from #ghc:

14:02 < mmorrow> weird:
14:02 < mmorrow> ghci> case undefined of (0::Word) -> undefined
14:02 < mmorrow> panics
14:02 < mmorrow> but
14:02 < mmorrow> ghci> case undefined of (0::Word32) -> undefined
14:02 < mmorrow> *** Exception: Prelude.undefined
14:02 < mmorrow> (and Word{8,16,32,64} all work)
14:03 < mmorrow> this works too
14:03 < mmorrow> case (0::Word) of _ -> undefined
[and]
14:03 < mmorrow> case (0::Word) of 0 -> undefined
14:05 < mmorrow> ghci> case 0 of (0::Word) -> undefined
14:05 < mmorrow> *** Exception: Prelude.undefined
14:05 < mmorrow> ghci> case undefined of (0::Word) -> undefined
14:05 < mmorrow> ghc: panic! (the 'impossible' happened)...

[axman6]

The full error message from the previous comment:

ghc: panic! (the 'impossible' happened)
   (GHC version 6.10.1 for i386-unknown-linux):
         schemeE(AnnCase).my_discr __word 0

[simonpj]

Ah. It seems that GHC's bytecode lacks an instruction for comparing Word values, for use when discriminating among case alternatives.

I'm going to assign this to Simon M to decide what to do:

• Add a suitable byte code, or
• Call a primop as a Haskell function (probably better; more general)

What about string literals too?

In general, it's bad that the bytecode generator just falls over; it'd be much better for it to say "I can't do X", and recover gracefully.

Simon

[simonmar]

There's no good reason for this not to work, but it's not a 1-line fix. I'll try to get around to it sometime.

[igloo]

Duplicated by #3431.

[simonmar]

Fixed

Fri Sep 18 06:32:04 PDT 2009  Simon Marlow <marlowsd@gmail.com>
  * implement case-on-Word in the byte code generator/interpreter (#2881)