id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5267	Missing type checks for arrow command combinators	peteg	ross	"Is this expected to work?

{{{
{-# LANGUAGE Arrows #-}
module T where

import Prelude
import Control.Arrow

t = proc () ->
     do rec x <- arr id <<< (| (arr id) (returnA -< x) |)
        returnA -< x

t' = proc x ->
     do x <- arr id <<< (| (arr id) (returnA -< x) |)
        returnA -< x

t'' = proc x ->
     do x <- arr id <<< (| (arr id) (returnA -< 3) |)
        returnA -< x

t''' = proc x -> arr id <<< (| (arr id) (returnA -< 3) |)
}}}

I get:

{{{
/tmp/T.hs:8:18:
    The type of the first argument of a command form has the wrong shape
      Argument type: t_tX
    In the command: (arr id) <<< ((|(arr id) ((returnA -< x))|))
    In a stmt of a 'do' expression:
        x <- (arr id) <<< ((|(arr id) ((returnA -< x))|))
    In a stmt of a 'do' expression:
        rec {x <- (arr id) <<< ((|(arr id) ((returnA -< x))|))}

/tmp/T.hs:12:14:
    The type of the first argument of a command form has the wrong shape
      Argument type: t_tG
    In the command: (arr id) <<< ((|(arr id) ((returnA -< x))|))
    In a stmt of a 'do' expression:
        x <- (arr id) <<< ((|(arr id) ((returnA -< x))|))
    In the expression:
      proc x -> do { x <- (arr id) <<< ((|(arr id) ((returnA -< x))|));
                     returnA -< x }

/tmp/T.hs:16:14:
    The type of the first argument of a command form has the wrong shape
      Argument type: t_tq
    In the command: (arr id) <<< ((|(arr id) ((returnA -< 3))|))
    In a stmt of a 'do' expression:
        x <- (arr id) <<< ((|(arr id) ((returnA -< 3))|))
    In the expression:
      proc x -> do { x <- (arr id) <<< ((|(arr id) ((returnA -< 3))|));
                     returnA -< x }

/tmp/T.hs:19:18:
    The type of the first argument of a command form has the wrong shape
      Argument type: t_k
    In the command: (arr id) <<< ((|(arr id) ((returnA -< 3))|))
    In the expression:
      proc x -> (arr id) <<< ((|(arr id) ((returnA -< 3))|))
    In an equation for `t'''':
        t''' = proc x -> (arr id) <<< ((|(arr id) ((returnA -< 3))|))
}}}
"	bug	new	low	7.6.2	Compiler (Type checker)	7.0.3			ross@… peteg	Unknown/Multiple	Unknown/Multiple	GHC accepts invalid program