The problem is in your string -> WriterT w m a. It needs to know what the
w is. It knows its a monoid but doesn't know anything else about it. The
simplest thing is to just tell it what it is.
foldrEntries :: (Entry -> a -> a) -> a -> (String -> a) -> Entries -> a
foldrEntries next done fail' es = runIdentity $ return . fst =<< runWriterT
foldIt
where
foldIt = foldrEntriesW (\e -> return . next e) (return done) ((return
:: Monad m => a -> WriterT () m a) . fail') es
A second option is to give foldIt a type signature. Unfortunately if you
want the a in foldIt to match the a in foldEntries, you have to use scoped
type variables extension. Normally the two signatures are not related and
the compiler figures both a's are not the same as each other.
{-# LANGUAGE ScopedTypeVariables #-}
...
foldrEntries :: forall a. (Entry -> a -> a) -> a -> (String -> a) ->
Entries -> a
foldrEntries next done fail' es = runIdentity $ return . fst =<< runWriterT
foldIt
where
foldIt :: Monad m => WriterT () m a
foldIt = foldrEntriesW (\e -> return . next e) (return done) (return .
fail') es
On Fri, Nov 20, 2015 at 7:14 AM, Martin Vlk
Hi I have two functions, foldrEntries and foldrEntriesW, where the latter is a WriterT version of the former. Here are the type signatures:
foldrEntries :: (Entry -> a -> a) -> a -> (String -> a) -> Entries -> a
foldrEntriesW :: (Monoid w, Monad m) => (Entry -> a -> WriterT w m a) -> WriterT w m a -> (String -> WriterT w m a) -> Entries -> WriterT w m a
I want to implement foldrEntries in terms of foldrEntriesW using the Identity monad and ignore/not use the writer result. I am doing this in order to reuse the foldrEntriesW implementation and avoid code duplication.
This is what I have so far: http://lpaste.net/145641
But the compiler complains about ambiguous type for the writer reult I am ignoring (message in the above lpaste).
Normally I think the way around this is to provide explicit type annotation for "foldIt", but in this case the result type depends on the type of "a" in foldrEntries type and I don't know how to express this and make the compiler happy.
(I was able to make it work by calling "tell ()", basically writing a dummy value, which lets compiler know what the type is, but this is not so good - I don't want to make artificial function calls like this.)
Can anybody help me with that?
Many Thanks Martin _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners