Hi
I've downloaded Hackmain from patch-tag, but I'm getting a different
error. The error I get is:
Hackmain.hs:63:10:
No instance for (Data.Typeable.Typeable2
Control.Monad.Reader.Reader)
arising from a use of `interpret' at Hackmain.hs:63:10-67
Hint requires the interpreted values to be an instance of Typeable in
order to check, in runtime, that the interpreted value matches the
type declared at compile. Therefore, you need to make sure that
(Filter a) is indeed an instance of Typeable.
Since you have Filter a = Reader (Config, Email) a, you probably need to
- Derive Config and Email instances for Filter,
- Manually provide Typeable instances for Reader a b, something along
the lines of:
instance (Typeable a, Typeable b) => Typeable (Reader a b) where...
(I don't know why this isn't done in the mtl....)
- Change the signature to:
getFilterMain :: (Typeable a, Deliverable a) => FilePath ->
Interpreter (Filter a)
Also, you can try using "infer" instead of "as :: ...."
Hope that helps
Daniel
On Mar 5, 2009, at 8:47 PM, Joseph Fredette wrote:
So, I tried both of those things, both each alone and together. No
dice. Same error, so I reverted back to the
original. :(
However, I was, after some random type signature insertions, able to
convert the problem into a different one, via:
getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter
a) getFilterMain MainLoc = do
loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/='.')
fMainLoc)] fMain <- (interpret
"(filterMain)" infer)
return (fMain :: Deliverable a => Filter a)
Inferred type is less polymorphic than expected
Quantified type variable `a' is mentioned in the environment:
fMain :: Filter a (bound at Hackmain.hs:77:1)
In the first argument of `return', namely
`(fMain :: (Deliverable a) => Filter a)'
In the expression: return (fMain :: (Deliverable a) => Filter a)
In the expression:
do loadModules [fMainLoc]
setTopLevelModules [(takeWhile (/= '.') fMainLoc)]
fMain <- (interpret "(filterMain)" infer)
return (fMain :: (Deliverable a) => Filter a)
I'm
thinking that this might be more easily solved -- I do think I
understand the issue. somehow, I need to tell the compiler
that the 'a' used in the return statement (return (fMain :: ...)) is
the same as the 'a' in the type sig for the whole function.
While I ponder this, and hopefully receive some more help -- thanks
again Dan, Ryan -- Are there any other options besides Hint that
might -- at least in the short term -- make this easier? I'd really
like to finish this up. I'm _so_ close to getting it done.
Thanks,
/Joe
Ryan Ingram wrote:
So, by using the Haskell interpreter, you're using the
not-very-well-supported dynamically-typed subset of Haskell. You can
tell this from the type signature of "interpret":
interpret :: Typeable a => String -> a -> Interpreter a
as :: Typeable a => a
as = undefined
(from
http://hackage.haskell.org/packages/archive/hint/0.2.1/doc/html/src/Language-Haskell-Interpreter-GHC.html)
In particular, the "as" argument to interpret is specifying what type
you want the interpreted result to be typechecked against; the
interpretation fails if it doesn't match that type. But you need the
result type to be an instance of Typeable; (forall a. Deliverable a
=>
Filter a) most certainly is not.
Off the top of my head, you have a couple of directions you can
take this.
(1) Make Typeable a superclass of Deliverable, saying that all
deliverable things must be dynamically typeable. Then derive
Typeable
on Filter, and have the result be of type "Filter a" using
ScopedTypeVariables as suggested before. (You can also pass "infer"
to
the interpreter and let the compiler try to figure out the result
type
instead of passing (as :: SomeType).)
(2) Make a newtype wrapper around Filter and give it an instance of
Typeable, and add a constraint to filterMain that the result type in
the filter is also typeable. Then unwrap the newtype after the
interpreter completes.
Good luck; I've never tried to use the Haskell interpreter before, so
I'm curious how well it works and what problems you have with it!
-- ryan
2009/3/5 Joseph Fredette <jfred...@gmail.com>:
I've been working on a little project, and one of the things I
need to do is
dynamically compile and import a Haskell Source file containing
filtering
definitions. I've written a small monad called Filter which is
simply:
type Filter a = Reader (Config, Email) a
To encompass all the email filtering. The method I need to import,
filterMain, has type:
filterMain :: Deliverable a => Filter a
where Deliverable is a type class which abstracts over delivery to
a path in
the file system. The notion is that I can write a type like:
data DEmail = {email :: Email, path :: FilePath}
newtype Maildir = MD DEmail
instance Deliverable Maildir where
{- ... omitted -}
However, Filter a should not be restricted to Deliverable types-
it also
encompasses the results of regular expression matching, etc, which
are not
-- in general -- Deliverable instances.
My question is this, when importing the file containing the
definitions of
filterMain, I have the following code to grab filterMain and
return it as a
function.
getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter
a)
getFilterMain fMainLoc = do
loadModules
[fMainLoc]; setTopLevelModules [(takeWhile (/='.') fMainLoc)]
fMain <- (interpret "(filterMain)" (as ::
Deliverable a
=> Filter a)) return (fMain)
However, when I try to
compile
this, I get the type error:
Hackmain.hs:70:43:
Ambiguous type variable `a' in the constraint:
`Deliverable a'
arising from a use of `getFilterMainStuff' at Hackmain.hs:
70:43-60
Probable fix: add a type signature that fixes these type
variable(s)
My understanding is that a type like "Foo a => Bar a" (where Foo
is a class
and Bar is a datatype) would simply restrict
the values of a to only those implementing Foo. But evidently I'm
wrong. Is
there a good (read, easy... :) ) fix to this?
Any help would be greatly appreciated.
/Joe
PS. All the actual code is on patch-tag, here
http://patch-tag.com/repo/Hackmail/home -- if anyone prefers to
look at that
directly, the relevant files are in Src, namely, Hackmain.hs,
Filter.hs, and
Deliverable.hs
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