Enable "rec" keyword when RecursiveDo is enabled?

GHC supports recursive statement groups marked by the keyword rec, however this is only activated via a hack. Consider this (silly) example program:

{-# LANGUAGE RecursiveDo, Arrows #-}

main :: IO ()

main =
 mdo x <- return (length [1..42::Int])
     rec b <- return x
         let a = const c
         c <- print "x"
     return (a b)

The Arrow language needs to be enabled, otherwise the rec keyword wouldn't be recognised. Currently this behaviour is undocumented, and rec groups only appear in the internal AST.

This ticket has been created to have a documented decsion whether this feature is supported or not. Given that GHC automatically identifies strongly-connected components and re-groups things (although it cannot re-order statements due to probable dependencies) having an explicet rec keyword probably isn't that useful. In particular

mdo foo   ...is equivalent to...      mdo rec foo
    bar                                       bar
    baz                                   baz

Thoughts?

Trac metadata

Trac field	Value
Version	6.11
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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