The 7.8-rc2 User Guide for rebindable syntax includes: Arrow notation (see Section 7.16, "Arrow notation ") uses whatever arr,
(>>>), first, app, (|||) and loop functions are in scope. But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask!
I want to use this: my types for the functions apparently do not match the Prelude types closely enough. Whom should I ask about this? Thanks very much. -Nick ----- Further Context If I do something similar with Monad and RebindableSyntax, it type-checks. I'm doing some experimentation for work. I'd like to index my arrows as follows. (Disclaimer: I have not yet considered any of the laws; I do not currently intend to support `app' and |||.) infixr 1 >>>, <<< type family (:*:) (i :: Maybe *) (j :: Maybe *) :: Maybe * type instance Nothing :*: j = j type instance i :*: Nothing = i type instance Just i :*: Just j = Just (i,j) returnA :: IArrow arr => arr Nothing a a returnA = arr id -- second, ***, and &&& can have the standard default definitions class IArrow (arr :: Maybe * -> * -> * -> *) where arr :: (b -> c) -> arr Nothing b c (>>>) :: arr i a b -> arr j b c -> arr (i :*: j) a c first :: arr i b c -> arr i (b,d) (c,d) Unfortunately, the following does not type-check with -XArrows and -XRebindableSyntax. diagram :: IArrow arr => arr f x fx -> arr g x gx -> arr (f :*: g) x gx diagram f g = proc x -> do fx <- f -< x gx <- g -< x returnA -< gx The error messages indicate that the type-checker is attempting to unify the types of f, g, returnA, and the entire proc block. However, the copy-and-pasted desugaring of that code *does* type-check (mutatis mutandi). This S data type is an example of an IArrow. data S i a b where SN :: ( a -> b ) -> S Nothing a b SJ :: s -> ((a,s) -> (b,s)) -> S (Just s) a b In terms of the S type, the objective of IArrow is to allow each "box in the diagram" to have its own state.