Option types and pattern matching.

[Jacob Carlborg via Digitalmars-d]

On 2015-10-26 12:40, Edmund Smith wrote:

> You could also emulate constant matching using default parameters
> (albeit with the restriction that they must be after any
> non-default/constant parameters), since the defaults form part of the
> function's type. I tried making something like this earlier this summer
> and it'd check that a given value was first equal to the default
> parameter and match if so, or match if there was no default parameter
> but the types matched.
>
> e.g.
> //template ma(tch/g)ic
>
> unittest
> {
>      Algebraic!(string, int, double, MyStruct) v = 5;
>      string s = v.match!(
>          (string s = "") => "Empty string!",
>          (string s) => s,
>          (int i = 7) => "Lucky number 7",
>          (int i = 0) => "Nil",
>          (int i) => i.to!string,
>          (double d) => d.to!string,
>          (MyStruct m = MyStruct(15)) => "Special MyStruct value",
>          (MyStruct m) => m.name, //
>          () => "ooer");
>      writeln(s);
> }
>
> It's a bit ugly overloading language features like this, but it makes
> the syntax a little prettier.

Why not just use a value as an extra argument:

v.match!(
     7, (int i) => "Lucky number 7"
);

> Scala's Option is really nice on the other hand since you can/should pattern match).

I thought it was preferred to use the higher order functions like "map" 
and "filter".

-- 
/Jacob Carlborg