I have a trick that loses a little convenience, but may still be more convenient than data families.
{-# LANGUAGE TypeFamilies #-} import Data.Tagged type family F a foo :: Tagged a (F a) foo = Tagged undefined bar :: Tagged a (F a) bar = foo This allows you to use the same newtype wrapper consistently, regardless of what the type instance actually is; one of the inconveniences of data families is the need to use different constructors for different types. On Sun, Jan 13, 2013 at 2:10 PM, Conal Elliott <co...@conal.net> wrote: > I sometimes run into trouble with lack of injectivity for type families. > I'm trying to understand what's at the heart of these difficulties and > whether I can avoid them. Also, whether some of the obstacles could be > overcome with simple improvements to GHC. > > Here's a simple example: > > > {-# LANGUAGE TypeFamilies #-} > > > > type family F a > > > > foo :: F a > > foo = undefined > > > > bar :: F a > > bar = foo > > The error message: > > Couldn't match type `F a' with `F a1' > NB: `F' is a type function, and may not be injective > In the expression: foo > In an equation for `bar': bar = foo > > A terser (but perhaps subtler) example producing the same error: > > > baz :: F a > > baz = baz > > Replacing `a` with a monotype (e.g., `Bool`) eliminates the error. > > Does the difficulty here have to do with trying to *infer* the type and > then compare with the given one? Or is there an issue even with type > *checking* in such cases? > > Other insights welcome, as well as suggested work-arounds. > > I know about (injective) data families but don't want to lose the > convenience of type synonym families. > > Thanks, -- Conal > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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