# Arrow Notation - Command combinators don't work with higher rank types? (GHC 7.4)

Ben Moseley ben_moseley at mac.com
Sun Jan 15 10:34:35 CET 2012

```I have a quick question on the current (GHCi, version 7.4.0.20111219) implementation of arrow notation.

Consider the code below:

----

{-# LANGUAGE Arrows,Rank2Types #-}

import Control.Arrow

-- cmdcomb :: Arrow a => (a (env,x) x) -> a (env,x) x
-- cmdcomb aegg = aegg

cmdcomb :: Arrow a => (forall x . a (env,x) x) -> a (env,x) x
cmdcomb aegg = aegg

myarr :: Arrow a => a (Int,Bool) Bool
myarr = proc (i,b) -> do
(|cmdcomb (\g -> returnA -< g) |) 'x'
-- (| (cmdcomb (arr snd)) |) 'x'
returnA -< False

----

This code generates the error below (but using either of the commented sections instead gets it to typecheck):

../MyDev/FPF3/saturday/dm.hs:13:13:
Couldn't match expected type `t0 t1 t2'
with actual type `forall x. a0 (env0, x) x'
Expected type: t0 t1 t2 -> a (a1, t4) t3
Actual type: (forall x. a0 (env0, x) x) -> a0 (env0, x0) x0
In the expression: cmdcomb
In the expression:
proc (i, b) -> do { (|cmdcomb ((\ g -> returnA -< g))|) 'x';
returnA -< False }