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?
It is due to the rules for the translation of list comprehensions: [ e | True ] = [ e ] [ e | q ] = [ e | q, True ] [ e | b, Q ] = if b then [ e | Q ] else [] [ e | p <- l, Q ] = let ok p = [ e | Q ] ok _ = [] in concatMap ok l [ e | let decls, Q ] = let decls in [ e | Q ] So [ value | boolean ] matches the second rule giving us [value | boolean, True] which matches the third rule if boolean then [value | True] else [] which can be simplified via the first rule to if boolean then [value] else [] These rules are slightly more complex than necessary because they avoid using a null base case. We could simplify the first two rules if we were to allow the degenerate list comprehension [ e | ] and let Q match nothing. Then we'd use the rule: [ e | ] = [ e ] and translate [ value | boolean ] via the original 3rd rule with Q as nothing: if boolean then [value | ] else [] and directly to: if boolean then [value ] else [] If you meant, why is it allowed rather than banned then I guess the answer is because it is orthogonal. The rules naturally handle that case and there was no particular reason to ban it, even if it is somewhat unusual. Duncan