[Haskell-cafe] generalized list comprehensions

[Jonathan Cast]

On Mon, 2008-11-10 at 18:20 +0000, Andrew Coppin wrote:
> Mitchell, Neil wrote:
> > In general:
> >
> > if boolean then [value] else []
> >
> > Can be written as:
> >
> > [value | boolean]
> >   
> 
> Is there any specific reason why this is valid?

Is there any specific reason to dis-allow it?  The grammar here looks
something like (NB: I didn't double-check the report):

    list_compr ::= [ value | generator* ]
    generator ::= boolean | pat <- list | let binds

One particular special case is where there is exactly one generator,
which has three further special cases:

    [ value | boolean ]
    [ value | pat <- expr ]
    [ value | let binds ]

These are all valid because they are special cases of the general list
comprehension syntax; the de-sugarings are all just special cases of the
general list comprehension de-sugaring rules:

      [ value | ] = [ value ]
      [ value | boolean, generators ]
    = if boolean then [ value | generators ] else []
      [ value | pat <- expr, generators ]
    = expr >>= \ x -> case x of pat -> [ value | generators ]; _ -> []
      [ value | let binds, generators ]
    = let binds in [ value | generators ]

So the special cases simplify to

      [ value | boolean ] = if boolean then [ value ] else []
      [ value | pat <- expr ]
    = expr >>= \ x -> case x of pat -> [ value ]; _ -> []
      [ value | let binds ] = let binds in [ value ]

Why wouldn't this work?

jcc