[Haskell-cafe] [weird stuff] The Dodgy Diagonal

Stefan O'Rear stefanor at cox.net
Sat Jul 14 15:37:44 EDT 2007

On Sat, Jul 14, 2007 at 12:06:30PM +0100, Conor McBride wrote:
> A peculiar query for folks who know more about the internals of Haskell
> compilers than I do. I attach the full code with all the bits and pieces,
> but let me pull out the essentials in order to state the problem.
> > newtype Id          x = Id x                   -- element
> > newtype K1 a        x = K1 a                   -- constant
> > data ((Sum1 p q))   x = L1 (p x) | R1 (q x)    -- choice
> > data ((Prod1 p q))  x = P1 (p x) (q x)         -- pairing
> > newtype Fst         x y = Fst x
> > newtype Snd         x y = Snd y
> > newtype K2 a        x y = K2 a
> > data ((Sum2 p q))   x y = L2 (p x y) | R2 (q x y)
> > data ((Prod2 p q))  x y = P2 (p x y) (q x y)
> > class (Bifunctor b, Functor f) => Diag b f | b -> f where
> >   diag :: b x x -> f x
> >   gaid :: f x -> b x x
> > instance Diag Fst Id where
> > instance Diag Snd Id where
> > instance Diag (K2 a) (K1 a) where
> > instance (Diag pb pf, Diag qb qf) => Diag (Sum2 pb qb) (Sum1 pf qf) where
> > instance (Diag pb pf, Diag qb qf) => Diag (Prod2 pb qb) (Prod1 pf qf) 

> That looks like a whole lot of doing very little. So, can I (in practice, 
> in this or that compiler) get away with...
> > dodgy :: Diag b f => b x x -> f x
> > dodgy = unsafeCoerce#
> > ygdod :: Diag b f => f x -> b x x
> > ygdod = unsafeCoerce#
> ...dodgy for diag and ygdod for giad?
> Minimal nontrivial experiments in ghc give grounds for cautious optimism,
> but I'd be delighted to hear from better informed sources.

In GHC 6.7.20070712 and Yhc, this is perfectly safe.  The "ABI" GHC uses for
data types is determined only by the number of constructors and the
number of pointer and non-pointer arguments to each constructor; so your
pairs of values are compatible.

Case analysis is done in these systems by examining a small integer
pointed to from the tag pointer, so identically-shaped data types will
have identical small integers

module X where
data X a b = Y a | Z a

moo z = case z of Z a -> a ; Y a -> a

A fragment of the asm output, rearranged and commented:

X_moo_info:             ; Enter here.  EBP is the stack pointer, z is
                        ; on the stack, as is a return address.
  movl (%ebp),%esi      ; Load z into ESI, which the standard calling
                        ; convention uses for the pointer to the object
                        ; under evaluation.
  movl $s80_info,(%ebp) ; Store a return address, so that we can
                        ; evaluate z.  Appears to re-use the stack space
                        ; required for the z argument
  jmp *(%esi)           ; Jump to z's evaluation code

s80_info:               ; z is evaluated now; the result of evaluation
                        ; is in ESI.
  movl (%esi),%eax      ; Fetch the raw tag of the value
  cmpw $1,-2(%eax)      ; -2(%eax) fetches the small integer associated
                        ; with the tag.  Compare it to 1.
  jae .Lc8g             ; degenerate decisiontree switch
  movl 4(%esi),%esi     ; Load the field of the evaluated constructor
  addl $4,%ebp          ; Clean up the stack
  jmp stg_ap_0_fast     ; Returning evals are tricky.  Jump to common
                        ; RTS code
.Lc8g:                  ; same as before
  movl 4(%esi),%esi
  addl $4,%ebp
  jmp stg_ap_0_fast

In GRIN based systems like Jhc, this is *not* safe, since after
evaluation comparisons are done using the full tag.

{-# OPTIONS_JHC --noprelude #-}
-- compiling the prelude would take days, I'm not going to wait
module Main where

import Jhc.Prim
import Jhc.Basics
import Jhc.Monad

data X1 a b = Y1 a | Z1 a
data X2 a b = Y2 a | Z2 a

main = case (unsafeCoerce__ (Z2 ())) of Y1 _ -> putStrLn "No"
                                        Z1 _ -> putStrLn "Yes"

putChar :: Char -> IO ()
putChar c = c_putwchar (ord c)

foreign import ccall "stdio.h jhc_utf8_putchar" c_putwchar :: Int -> IO ()

putStr     :: String -> IO ()
putStr s   =  mapM_ putChar s

putStrLn   :: String -> IO ()
putStrLn s =  do putStr s
                 putChar '\n'

stefan at stefans:/usr/local/src/jhc$ ./hs.out 
match falls off bottom: Main.Z2 ()

I don't know enough about nhc, hbc, yale-haskell, or Hugs to answer for

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