On 3/19/06, Manuel M T Chakravarty
Loosely related to Ticket #76 (Bang Patterns) is the question of whether we want the language to include strict tuples. It is related to bang patterns, because its sole motivation is to simplify enforcing strictness for some computations. Its about empowering the programmer to choose between laziness and strictness where they deem that necessary without forcing them to completely re-arrange sub-expressions (as seq does).
So what are strict tupples? If a lazy pair is defined in pseudo code as
data (a, b) = (a, b)
a strict pair would be defined as
data (!a, b!) = ( !a, !b )
Ie, a strict tuple is enclosed by bang parenthesis (! ... !). The use of the ! on the rhs are just the already standard strict data type fields.
Maybe I've missed something here. But is there really any reasonable usage cases for something like: f !(a,b) = a + b in the current bang patterns proposal? I mean, would anyone really ever want an explicitly strict (i.e. using extra syntax) tuple with lazy elements? Couldn't the syntax for strict tuples be just what I wrote above (instead of adding weird-looking exclamation parenthesis). I'm pretty sure that most programmers who would write "f !(a,b) = ..." would expect the tuple's elements to be forced (they wouldn't expect it to do nothing, at least).. In fact !(x:xs) should mean (intuitively to me, at least) "force x, and xs", meaning that the element x is forced, and the list xs is forced (but not the elements of the xs). Couldn't this be generalised? A pattern match on any constructor with a bang in front of it will force all the parts of the constructor (with seq)? So: f !xs = b -- gives f xs = xs `seq` b, like the current proposal f !(x:xs) = b -- gives f (x:xs) = x `seq` xs `seq` b, unlike the current proposal? The latter would then be equal to f (!x:xs) = b right? Just slightly more convenient in some cases... f (A !b (C !c !d) !e !f !g) = y would be equivalent to: f !(A b !(C c d) e f g) = y Instead of the latter meaning doing nothing... /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862