[Haskell-beginners] Ambiguous type of WriterT result I am not using

Martin Vlk martin at vlkk.cz
Fri Nov 20 12:14:25 UTC 2015

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 ->

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:

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

More information about the Beginners mailing list