On Monday, 21 March 2016 at 11:50:06 UTC, Timothee Cour wrote:
On Mon, Mar 21, 2016 at 4:34 AM, Nick Treleaven via
Digitalmars-d < digitalmars-d@puremagic.com> wrote:
On 14/03/2016 11:32, thedeemon wrote:
filter_map : ('a -> 'b option) -> 'a t -> 'b t
"filter_map f e returns an enumeration over all elements x
such as f y returns Some x, where y is an element of e."
It is really convenient and comes handy in many situations.
However it requires some common Option/Maybe type that
different libraries could use.
There is a pull for Option:
https://github.com/D-Programming-Language/phobos/pull/3915
We could have:
// fun takes r.front and produces an Option of that type
auto mapFilter(alias fun, R)(R r);
// turn a possibly null value into an Option
Option!T nullFilter(T)(T v) if (isNullable!T);
auto src = [new Object(), new T(), null];
auto res = mapFilter!(e => nullFilter(cast(T)e));
assert(res.equal([src[1]]));
see my proposal [+implementation] for emit
http://forum.dlang.org/post/mailman.538.1458560190.26339.digitalmar...@puremagic.com
emit is more powerfull, and generalizes map,filter,joiner
auto res = src.mapFilter!(e=>nullFilter(cast(T)e));
with emit:
auto res = src.emit!((put,e){if(cast(T)e) put(e);});
Why don't you go for the well-established monadic bind function ?
A rangified bind would take a range of inputs, a lambda returning
a range of results for one input and return a range of all
results.
It is logically just a combination of map and concat (which turns
a range of ranges into a combined range, but I think that one is
missing in the std lib too).
