On Feb 6, 2006, at 19:33, Brian Hulley wrote:
Clean allows (AFAIK) several distinctions to be made:
1) ![a] means that the list of a's is a strict argument, just like writing !b
2) [!a] means that the list is head strict (List1 a)
3) [a!] means that the list is tail strict (List2 a)
4) [!a!] means that the list is head and tail strict (List3 a)
5) ![!a!] means that the head-and-tail-strict-list-argument is strict!!!
Right. I think it's worth stressing that the four possible list types are all different as far as type checking is concerned. There's special list syntax however to denote lists that are overloaded in the list type. At run time the different list types share the same nil and cons constructors. This means that conversions between list types are often cheap. I have a small library that does this, with the following costs (monospaced table): \to \ [ ] [! ] [ !] [!!] frm\ [ ] 0 c e! !e! [! ] 0 0 e! e! [ !] 0 c 0 !e [!!] 0 0 0 0 0 = doesn't traverse list e = traverses list (evaluating elements (!e), spine (e!), or both (! e!)) c = copies list (lazily, evaluating elements)
I think also (though I'm not entirely sure) that these distinctions are generalized for other data types by talking about element strictness and spine strictness.
No, there's no such generalisation. Cheers, Ronny Wichers Schreur