On 2008 May 18, at 9:59, Kaveh Shahbazian wrote:
For something like: type Thing a b = ThisWay a | ThatWay b | NoWay actually there is no equivalents for data constructor
I presume you mean "data" instead of "type". Not that I can address your question directly, as I don't know C#. In C it's a union; in C++ you would export constructors ThisWay(), ThatWay(), NoWay() from class Thing. I presume C# is similar. Haskell syntax is decidedly more compact than any of them.
in C# 3 (I think). I asked it if there are other ideas about this: Controlling the execution path by deciding based of structure of data with a trick other than reflecting!
in C-like languages, the class includes a structure tag which is set by the constructor. Guess what? That's how Haskell does it, just implicitly (hence, again, nicely compact syntax). (You can actually experiment with this in GHC; using internal stuff like UnsafeCoerce# you can coerce one datum to another if they have the same (0-based, assigned in order of definition) constructor tag and the same representation for the data value. IIRC in GHC the internal constructor tag is an 8-bit unsigned value.) To make the above a bit clearer (I hope), here's a rough C approximation of a simple Haskell type: /* data Foo = FooInt Int | FooDouble Double */ struct Foo { unsigned char _FooTag; union { #define _FooTag_FooInt 0 int _FooInt; #define _FooTag_FooDouble 1 double _FooDouble; } _Foo_u; }; /* here I assume unboxed basic types for simplicity */ struct Foo *FooInt(int param) { Foo *foo = malloc(sizeof *foo); /* assume sane error checking here */ foo->_FooTag = _FooTag_FooInt; foo->_Foo_u._FooInt = param; } struct Foo *FooDouble(int param) { Foo *foo = malloc(sizeof *foo); /* assume sane error checking here */ foo->_FooTag = _FooTag_FooDouble; foo->_Foo_u._FooDouble = param; } /* * bar (FooInt i) = ... * desugars to * bar x = case x of { FooInt i -> ... } * which is very roughly (pretend I catch x == 0 and invoke error("Undefined")) * bar(Foo x) { switch (x->_FooTag) { case _FooTag_FooInt: i = x->_Foo_u._FooInt; ... } } */ -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH