[Haskell-cafe] ambiguous constraint errors

Evan Laforge qdunkan at gmail.com
Wed May 28 21:29:23 EDT 2008

I have two related questions:


I'm getting some annoying type errors that I don't fully understand,
and wind up having to do a workaround that I don't totally like.
Here's a simplified version of my situation:

data Ambi m = Ambi {
    ambi_monad :: m Int
    , ambi_int :: Int

some_ambi :: Monad m => Ambi m
some_ambi = Ambi (return 5) 10

ambi_table :: Monad m => [(String, Ambi m)]
ambi_table = [("default", some_ambi)]

get_int :: String -> Maybe Int
get_int sym = fmap ambi_int (lookup sym ambi_table)


get_int produces:
    Ambiguous type variable `m' in the constraint:
      `Monad m' arising from a use of `ambi_table' at ambi.hs:13:40-49

So I guess this means I'm not telling it which 'm', so it doesn't know
how to resolve the 'return'... but the thing is, I'm not even using
that value, so it doesn't matter what it resolves to.  So it works if
I pick some random monad:

get_int sym = fmap ambi_int (lookup sym ambi_table :: Maybe (Ambi Maybe))

Note that I can't leave it as 'Monad m => Ambi m' because I still get
an ambiguous type variable complaint.

I'm a little disconcerted by having to pick some random dummy monad.
Even worse, everything this type touches starts requiring explicit
type declarations everywhere.  Is there some easier way to do this?


This is somewhat related to another issue I've been having, which is
that I have some kind of complicated type, e.g. '(SomeMonad some,
Monad m) => some (SomethingM m Status)' that I use in a lot of places.
 It would be a lot less typing and easier to modify later if I wrote a
type alias:

type Command = (Monad some, Monad m) => some (State.StateT () m Status)

but of course, this isn't allowed, since the type variables don't
appear on the lhs, and if I put a context there, it's a syntax error.
While I can write it with data:

data (Monad some, Monad m) => Command some m = Command (some
(State.StateT () m Status))

I've been told this doesn't mean what I expect it to, which is that
the context constraints propagate up to and unify with the containing
type (out of curiosity, since it's accepted, what *does* this do?  I
think I read it somewhere once, but now I forget and can't find it).
And sure enough, using this type doesn't make my type declarations
have the right contexts.

So the first problem means that I have to declare types in various
inconvenient places, and the second one means that I have to type out
all the various class constraints (I can still alias away the
non-polymorphic bits), and all my type declarations start looking much
more complicated than they are.

The "solution" I've been using for some of this is just to remove the
polymorphism, so I can write a simple alias like

type Command = SomethingM (State.StateT () Identity Status)

and now I can think of "a command" and have various functions that
take and return Commands, without caring that it's some kind of monad
with context constraints.  But of course, this isn't always possible
since sometimes I need the type to remain polymorphic (i.e. while most
of these I don't *think* will run in some other monad, some of them
definitely get called in multiple contexts).

Is there any nicer way around this?  And what's the underlying issue
that makes this necessary?  I can live with all the context hair
everywhere, but it sure would be nicer to be able to define it once
and for all in one place.

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