digitalmars.D - [Request] A way to extract all instance of X from a range

deadalnix (4/4) Mar 14 2016 Right now, I'm repeating the following pattern many times :

ZombineDev (20/24) Mar 14 2016 BTW, .NET has an extension method called OfType that does the
thedeemon (15/17) Mar 14 2016 This reminds me of a function in OCaml extended stdlib that I've

Nick Treleaven (11/16) Mar 21 2016 There is a pull for Option:

Timothee Cour via Digitalmars-d (8/28) Mar 21 2016 see my proposal [+implementation] for emit

Matthias Bentrup (8/46) Mar 22 2016 Why don't you go for the well-established monadic bind function ?

Nick Treleaven (2/5) Mar 23 2016 http://dlang.org/phobos/std_range.html#.chain

Marc =?UTF-8?B?U2Now7x0eg==?= (3/9) Mar 23 2016 Or joiner(), if you have a range of ranges:

Jakob Ovrum (5/15) Apr 01 2016 Using the interface proposed in the PR:

Stefan Koch (3/7) Mar 14 2016 you could use an alias.
Stefan Koch (4/8) Mar 14 2016 There you go ;)

Stefan Koch (3/13) Mar 14 2016 Ooops ;) didn't work
Meta (4/14) Mar 14 2016 I believe this should work with the latest DMD:

Stefan Koch (12/15) Mar 15 2016 No it does not.

Meta (3/20) Mar 15 2016 What version of DMD are you using? It compiles and a small test

deadalnix <deadalnix gmail.com> writes:

Right now, I'm repeating the following pattern many times :

range.map!(x => cast(Foo) x).filter!(x => x !is null)

Which is kind of annoying. Could we get something in phobos to do 
this ?

Mar 14 2016

ZombineDev <petar.p.kirov gmail.com> writes:

On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

 Which is kind of annoying. Could we get something in phobos to 
 do this ?

BTW, .NET has an extension method called OfType<T> that does the 
same thing:
https://msdn.microsoft.com/library/bb360913(v=vs.100).aspx
which is different from Cast<T>, which throws an exception if an 
element can't be converted:
https://msdn.microsoft.com/library/bb341406(v=vs.100).aspx

How do you want to handle this? With two separate methods as in 
.NET, or with a template parameter?

Maybe something like this:

/// Returns a range that lazily iterates over
/// the elements in `source` and converts each
/// of them to type `T`.
///
/// If `throwOnConvError` is false, elements
/// that can't be converted will be skipped.
auto ofType
     (T, bool throwOnConvError = true, Range)
     (Range source)
     if (isInputRange!Range)

Mar 14 2016

thedeemon <dlang thedeemon.com> writes:

On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

This reminds me of a function in OCaml extended stdlib that I've 
used quite often and really miss in D:

filter_map : ('a -> 'b option) -> 'a list -> 'b list

"filter_map f l call (f a0) (f a1).... (f an) where a0..an are 
the elements of l. It returns the list of elements bi such as f 
ai = Some bi (when f returns None, the corresponding element of l 
is discarded)."

Or its variant for OCaml's analog of ranges:

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.

Mar 14 2016

Nick Treleaven <ntrel-pub mybtinternet.com> writes:

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]]));

Mar 21 2016

Timothee Cour via Digitalmars-d <digitalmars-d puremagic.com> writes:

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.digitalmars-d 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);});

Mar 21 2016

Matthias Bentrup <matthias.bentrup googlemail.com> writes:

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.digitalmars-d 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).

Mar 22 2016

Nick Treleaven <ntrel-pub mybtinternet.com> writes:

On Tuesday, 22 March 2016 at 20:09:51 UTC, Matthias Bentrup wrote:
 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).

Mar 23 2016

Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:

On Wednesday, 23 March 2016 at 11:36:39 UTC, Nick Treleaven wrote:
 On Tuesday, 22 March 2016 at 20:09:51 UTC, Matthias Bentrup 
 wrote:
 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).



Or joiner(), if you have a range of ranges:

Mar 23 2016

Jakob Ovrum <jakobovrum gmail.com> writes:

On Monday, 21 March 2016 at 11:34:15 UTC, Nick Treleaven wrote:
 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]]));

Using the interface proposed in the PR:

auto src = [new Object(), new T(), null];
assert(src.map!(e => 
option(cast(T)e)).joiner.equal(only(src[1])));

Apr 01 2016

Stefan Koch <uplink.coder googlemail.com> writes:

On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

 Which is kind of annoying. Could we get something in phobos to 
 do this ?

you could use an alias.

alias NullFliter = filter!(x => x !is null);

Mar 14 2016

Stefan Koch <uplink.coder googlemail.com> writes:

On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

 Which is kind of annoying. Could we get something in phobos to 
 do this ?

There you go ;)

alias castRange(T) =  map!(x => cast(T) x).filter!(x => x !is 
null);

Mar 14 2016

Stefan Koch <uplink.coder googlemail.com> writes:

On Monday, 14 March 2016 at 23:34:37 UTC, Stefan Koch wrote:
 On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

 Which is kind of annoying. Could we get something in phobos to 
 do this ?

 There you go ;)

 alias castRange(T) =  map!(x => cast(T) x).filter!(x => x !is 
 null);

Ooops ;) didn't work
then it'll has to be done with a wrapper struct ...

Mar 14 2016

Meta <jared771 gmail.com> writes:

On Monday, 14 March 2016 at 23:34:37 UTC, Stefan Koch wrote:
 On Monday, 14 March 2016 at 08:04:18 UTC, deadalnix wrote:
 Right now, I'm repeating the following pattern many times :

 range.map!(x => cast(Foo) x).filter!(x => x !is null)

 Which is kind of annoying. Could we get something in phobos to 
 do this ?

 There you go ;)

 alias castRange(T) =  map!(x => cast(T) x).filter!(x => x !is 
 null);

I believe this should work with the latest DMD:

alias castRange(T) = t => t.map!(x => cast(T) x).filter!(x => x 
!is null);

Mar 14 2016

Stefan Koch <uplink.coder googlemail.com> writes:

On Tuesday, 15 March 2016 at 04:05:11 UTC, Meta wrote:
 I believe this should work with the latest DMD:

 alias castRange(T) = t => t.map!(x => cast(T) x).filter!(x => x 
 !is null);

No it does not.
However this works :

	auto typeFilter(T, Range)(Range range) {
		import std.algorithm : filter, map;
		import std.traits : ForeachType;
		static assert(is(T == class) && is(T : ForeachType!Range),
			"typeFilter only works with classes that are derived form each 
other");
		return range.map!(x => cast(T) x).filter!(x => x !is null);
	}

You're Welcome.

Mar 15 2016

Meta <jared771 gmail.com> writes:

On Tuesday, 15 March 2016 at 07:42:58 UTC, Stefan Koch wrote:
 On Tuesday, 15 March 2016 at 04:05:11 UTC, Meta wrote:
 I believe this should work with the latest DMD:

 alias castRange(T) = t => t.map!(x => cast(T) x).filter!(x => 
 x !is null);

 No it does not.
 However this works :

 	auto typeFilter(T, Range)(Range range) {
 		import std.algorithm : filter, map;
 		import std.traits : ForeachType;
 		static assert(is(T == class) && is(T : ForeachType!Range),
 			"typeFilter only works with classes that are derived form 
 each other");
 		return range.map!(x => cast(T) x).filter!(x => x !is null);
 	}

 You're Welcome.

What version of DMD are you using? It compiles and a small test 
works for me on 2.070.2.

Mar 15 2016

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digitalmars.D - [Request] A way to extract all instance of X from a range