On Thu, 30 Oct 2014, Simon Peyton Jones wrote:
No no! Let's not do that. It's grotesque to generate identical code
twice. We must find a better way.
So the type of an open-kinded matcher function, for a pattern of type
pattern type P :: [T a]
would need to be something like
$m?P :: forall (r :: ?) a. [T a] -> R(r) -> R(r) -> r
where R(r) = Void# -> r if r :: #
, r otherwise
Is there a way to do that? I couldn't think of anything better than to
generate two versions:
$mP :: forall r a. [T a] -> r -> r -> r
$m#P :: forall (r :: #) a. [T a] -> (Void# -> r) -> (Void# -> r) -> r
Now, to cut down on the amount of code generated, I guess we could have
$m?P :: forall (r :: ?) a. [T a] -> (Void# -> r) -> (Void# -> r) -> r
and always compile pattern synonym match continuations into lambdas over
this dummy Void#, but I thought we also wanted to avoid that...
Note that if P were to have arguments, the same problem would still be
present with the fail continuation (but not the success one).
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