In compound formats, add special delimiters for the last pair

[Bolpat]

The compound format has two forms: %(…%) and %(…%|…%), the latter allowing to specify the delimiter precisely. However, rather commonly, the delimiter must be different for the last pair of elements. Let’s allow for 2 or 3 delimiters to handle the last pair, i.e. %(…%|d₀%|d₁%) and %(…%|d₀%|d₁%|d₂%) such that d₀ is used between all elements but the last pair. For the last pair, use d₂ if it’s the only pair, otherwise use d₁. If d₂ isn’t given, it’s as if it were identical to d₁.

Example:

foreach (friends; [["John", "Nancy"], ["Amy", "Joe", "Taylor"]])
{
    writefln("My friends are %-(%s%|, %).", friends);
    // My friends are John, Nancy.
    // My friends are Amy, Joe, Taylor.

    writefln("My friends are %-(%s%| and %).", friends);
    // My friends are John and Nancy.
    // My friends are Amy and Joe and Taylor.

    // Proposed: Two %|
    writefln("My friends are %-(%s%|, %| and %).", friends);
    // My friends are John and Nancy.
    // My friends are Amy, Joe and Taylor.

    // Proposed: Three %|
    writefln("My friends are %-(%s%|, %|, and %| and %).", friends);
    // My friends are John and Nancy.
    // My friends are Amy, Joe, and Taylor.
}

[crazymonkyyy]

it seems insane to me to glue on more pieces to old c api's and would suggest trying to get a "look ahead" range concept of some kind, merged.

Fundamentally you are "looking ahead" by 1 element, slide(2) can be implement with 1 element of space and your just managing the complexity of the last element and bad tuple handling. You dont want to do this with just creating english sentences, all of parsing needs something like it like "is this an escaped symbol" "is this a < or a <=" "this seems numbery is this a int or a float".

[pbackus]

Format strings are meant to be a concise syntax for simple string formatting, not a general-purpose tool that covers every use-case. If a user wants more control over formatting than format strings offer, they can write normal D code to implement whatever custom formatting behavior they desire.

[crazymonkyyy]

@pbackus I dont really think this is well handled the available algorithms; I dont see how you handle this case cleanly without asking end users to know how to write look ahead with ranges

import std;
template switchmap(alias router,tracks...){
	auto switchmap(R)(R r){
		struct map{
			R r;
			auto front(){
				switch(router(r)){
					static foreach(I;0..tracks.length){
						case I:return tracks[I](r.front);
					}
					default: assert(0,"router overflowed");
			}}
			void popFront(){r.popFront;}
			bool empty()=>r.empty;
		}
		return map(r);
}}
unittest{
	foreach (friends; [["no one"],["John", "Nancy"], ["Amy", "Joe", "Taylor"]]){
		friends.switchmap!(r=>r.length.min(3)-1,
			s=>s,
			s=>s~" and ",
			s=>s~", ")
		.join.writeln;
}}