wolfgang.thaller:
> >g (F# f) =
> > let w = W32# (unsafeCoerce# f)
>
> Why does GHC even accept this code?
> I think unsafeCoerce# is not intended to be able to coerce unboxed
> values.
>
> Prelude GHC.Base> :t unsafeCoerce#
> unsafeCoerce# :: forall b a. a -> b
>
> The type variables a and b are supposed to be of kind *, and f is of
> kind #.
How do you know the tyvars have kind * ? Anyway, it's even used like
this in the libraries :)
GHC.Conc:
mkWeak key val Nothing = IO $ \s ->
case mkWeak# key val (unsafeCoerce# 0#) s of
{ (# s1, w #) -> (# s1, Weak w #) }
So those tyvars must really be ? kind (I think). I.e.
forall b:? a:? . a -> b
or
forall b:?? a:?? . a -> b
There's a description of this stuff in types/Kind.lhs.
Also, I see in basicTypes/MkId.lhs:
unsafeCoerce# isn't so much a PrimOp as a phantom identifier, that
just gets expanded into a type coercion wherever it occurs. Hence we
add it as a built-in Id with an unfolding here.
The type variables we use here are "open" type variables: this means
they can unify with both unlifted and lifted types. Hence we provide
another gun with which to shoot yourself in the foot.
open == ??, I think, and the above comment explains the unsafeCoerce# behaviour
I'm using/shooting myself with. Further down:
ty = mkForAllTys [openAlphaTyVar,openBetaTyVar]
(mkFunTy openAlphaTy openBetaTy)
which looks like: a:?? -> b:?? to me. In prelude/TysPrim.lhs:
-- openAlphaTyVar is prepared to be instantiated
-- to a lifted or unlifted type variable. It's used for the
-- result type for "error", so that we can have (error Int# "Help")
openAlphaTy = mkTyVarTy openAlphaTyVar
So I think this all implies that unsafeCoerce# x# is legal, though not polite.
Cheers,
Don
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