Friday, March 23, 2007

Lazy local pattern matching

While the upcoming release has a lot to do with improving the CAL Eclipse Plugin, we continue to work on the core framework, and even language features. One such language feature that should be putting in an appearance soon is called "lazy local pattern matching".

Both current methods of decomposing values (case expresssions and data constructor field selection expressions) cause a value to be evaluated to weak head normal form (WHNF) - i.e. so the top level constructor of the value is known. The new syntax will allow pattern matching on the LHS of a LET definition e.g.:

// list cons patterns:
public foo3 = let
a:b = [3];

// tuple patterns:
public foo4 = let
(a, b, c) = (b, c, 1.0);

// record patterns:
public foo5 = let
{a} = {a = "foo"};
public foo6 = let
{_ | a} = {a = "foo", b = "bar"};

The difference between this syntax and case or field selection expressions is that the RHS of the LET definition is not evaluated until some value in the LHS is evaluated.

More details on this feature will be published soon.

1 comment:

Anonymous said...

Hello. I really think that such sites like yours are very important for self-development for everybody. Thank you for yours work.