Here's a different example:
foo = let data Foo = Foo in [Foo, Foo]
Since unfolding let bindings is always sematically equivalent
to the original expression this must be the same as
foo = [let data Foo = Foo, let data Foo = Foo]
How will you make that work? A, you could outlaw unfolding
'data', but then you would have to outlaw unfolding any expression
with an embedded 'data'. Very ugly. Or B, you can go for
structural equality on types. But that would be a radical departure
from Haskell. (BTW, this kind of example is the very reason Cayenne
has structural equality on types by default.)
I don't think local (scoped) type definitions mesh at all well
with the Haskell design. This work nicely now because type definitions
are only allowed at the top level, so they have a definition location.
Which means that name equality works.
-- Lennart
Quoting Christophe Poucet <[EMAIL PROTECTED]>:
Dear,
Yesterday, while discussing with Cale and SamB on I suddenly came up with
the crazy idea of scoped data declarations. After some brief discussion to
check the validity, I finally came to the conclusion that they should be
feasible. In addition, I don't think that they would require a high amount
of changes in current compilers.
Basically if you have something like:
module Main where
foo = let data Foo = Foo deriving Show in Foo\
main :: IO ()
main = print foo
One can see this as having an extra hidden module that defines Foo but that
does not export it. The only change that is then required is that while
compiling Foo, the hidden-ness of Foo must be removed.
For instance, if one were to load this into, say, ghci (this is fictive of
course):
# ghci Main.hs
:t foo
foo :: Codeloc2.Foo
There were initially some objections to this, because it is no longer
feasible to actually write the type of the function foo. But if one looks
at current GHC, this objection is already there:
module A(foo) where
data Foo = Foo deriving Show
foo = Foo
module Main where
import A
main = print foo
As Excedrin then pointed out, importing this Main into ghci, gives
foo :: Foo.Foo
And this notation can not be written in Main either, because Foo is hidden
in A.
Therefore, I would like to note that scoped data declarations are just like
hidden data-declarations with two extra requirements:
1) Generate source-location-based submodule names
2) Add an extra import rule for those hidden modules in the subexpressions
of where the data-declaration is being originally defined.
Comments are welcome, of course :)
Cheers!
Christophe (vincenz)
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