On Wed, 2008-08-27 at 13:34 -0700, Aaron Tomb wrote:
> On Aug 27, 2008, at 12:23 PM, Dan Weston wrote:
>
> > Huh? Type safety buys you not having to worry about dereferencing
> > stale nonnull pointers (lifetime of reference exceeding lifetime of
> > referent), but nothing about dereferencing null pointers, which are
> > the moral equivalent of Nothing.
>
> What type safety buys you, in my mind, is that Nothing is only a valid
> value for explicit Maybe types. In cases where you don't use Maybe,
> the "null" situation just can't occur. In languages with null
> pointers, any pointer could possibly be null.
>
> When you do use Maybe, you have to explicitly handle the Just and
> Nothing cases separately. Pattern matching reminds you that both are
> possible. I tend to view fromJust as a function you should only use if
> you're _very_, _very_ sure that the Nothing case is impossible. But,
> if so, why are you using a Maybe type in the first place?
Consider:
substType v ty x = fromJust $
do
TyVar s <- cast x
guard $ v == s
cast ty
`mplus` do
ForAll s ty' <- cast x
guard $ v == s
cast $ ForAll s ty'
`mplus` do
ForAll s ty' <- cast x
guard $ s `Set.member` freeTV ty
let
v' = Set.findMax (freeTV ty `Set.union` freeTV ty') ++
"1"
ty'' = substType v ty $ substType s (TyVar v') $ ty'
cast $ ForAll v' ty''
`mplus` do
ForAll s ty' <- cast x
cast $ ForAll s $ substType v ty ty'
`mplus` do
TyLambda s ty' <- cast x
guard $ v == s
cast $ TyLambda s ty'
`mplus` do
TyLambda s ty' <- cast x
guard $ s `Set.member` freeTV ty
let
v' = Set.findMax (freeTV ty `Set.union` freeTV ty') ++
"1"
ty'' = substType v ty $ substType s (TyVar v') $ ty'
cast $ TyLambda v' ty''
`mplus` do
TyLambda s ty' <- cast x
cast $ TyLambda s $ substType v ty ty'
`mplus` do return $ gmapT (substType v ty) x
ForAll :: String -> Type -> Type
TyLambda :: String -> Expr -> Expr
The last `case' is a catch-all, so you do know the result of the mplus's
is a Just, but you still need the Maybe.
jcc
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