Hello, Consider the following program: ------------------------------------------------------------------- -- bar.hs {-# LANGUAGE NoMonomorphismRestriction #-} data Status = Foo | Bar data Rec m a = Rec { get :: m a , status :: Status } defRec :: (Monad m) => Rec m a defRec = undefined myRec :: (Monad m) => Rec m a myRec = Rec x y where Rec x y = defRec ------------------------------------------------------------------- It doesn't compile (under GHC 7.10.2), the error is: bar.hs:16:7: No instance for (Monad t0) The type variable ‘t0’ is ambiguous When checking that ‘y’ has the inferred type y :: Status Probable cause: the inferred type is ambiguous In an equation for ‘myRec’: myRec = Rec x y where Rec x y = defRec Failed, modules loaded: none. If you remove the language extension, it does. Keeping the NoMonomorphismRestriction, it can be forced to typecheck by putting in a "dummy" type that satifies the constraint when obtaining the field `status` (that doesn't depend on `m`. ------------------------------------------------------------------- -- bar.hs {-# LANGUAGE NoMonomorphismRestriction #-} data Status = Foo | Bar data Rec m a = Rec { get :: m a , status :: Status } defRec :: (Monad m) => Rec m a defRec = undefined myRec :: (Monad m) => Rec m a myRec = Rec x y where s :: Rec Maybe a -> Status s = status x = get defRec -- y = s defRec ------------------------------------------------------------------- So, what's going on here? Is this a feature of this extension? It seems like the ability to destructure and then recombine is pretty fundamental, and it shouldn't break in the way that it does. (I.e. it's kinda crazy to have to put in a dummy type satisfying the constraint; is there some way to pass down the error -- Noon Silk, ن https://silky.github.io/ "Every morning when I wake up, I experience an exquisite joy — the joy of being this signature."