[Haskell-cafe] Implicit concatenation in list comprehensions

[Dan Weston]

Bulat Ziganshin wrote:
> Hello Neil,
> 
> Tuesday, July 21, 2009, 1:26:55 PM, you wrote:
> 
>>>  ++ [ "-i"      | not (null (ghcOptSearchPath opts)) ]
>>>  ++ [ "-i", dir | dir <- ghcOptSearchPath opts ]
> 
>> Following the discussions, I now support this extension too - I keep
>> seeing more and more places in my code where it would be very useful.
> 
>>>  ++        [ "-i"        | not (null (ghcOptSearchPath opts)) ]
>>>  ++ concat [ ["-i", dir] | dir <- ghcOptSearchPath opts ]

  [a   | c ] = concat $ do { c; return [a] }
  [a,b | c ]  = concat $ do { c; return [a,b] }

This would mean that

  [   | c ] = concat $ do { c; return [] }

The right is legal Haskell and gives []. The left is (not yet) legal. 
Should it be?

Dan