[Proposal] Loops: improve first-item-is-special scenario

Problem

I can’t count the times when I had to write something like

var first = true;
foreach (var x in xs) {
    if (!first) {
        builder.Append(",");
    }
    else {
        first = false;
    }    
    ...
}

Not only is this verbose, but also inefficient – each iteration checks the variable even though it is known in advance it would be false for all but the first.

Potential solutions

A. Provide a first {} block.

Example:

foreach (var x in xs) {
    first {} else {
        Append(",");
    }
    ...
}

Pros:

1. Reduced verbosity
2. Potential for compiler optimizations by separating first item handling from the remaining loop

Cons:

1. No idea for good not-first syntax: first {} else {}, !first {}?

B. Provide an automatic loop index variable in loops.

Example:

foreach (var x in xs) {
    if (index > 0) { // "index" is automatically provided here
        Append(",");
    }
    ...
}

Pros:

1. Reduced verbosity
2. An index that might be useful for other things such as numbering in output

Cons:

1. Compiler optimizer would have to look for specific coding patterns (if (index > 0)) if it wants to make the loop more efficient.

[mysticatea]

var en = xs.GetEnumerator();
if (en.MoveNext())
{
    var x = en.Current;
    // do for the first.

    while (en.MoveNext())
    {
        // do for not last items.
        x = en.Current;
        // do for not first items.
    }

    // do for the last. (`x` is last item.)
}
else {
    // do when empty.
}

[McZosch]

… in which case you can still use the indexed signature of the Select extension, write your own extension or simply convert to a in-memory representation, which is better when handling data in multi-threaded environments anyway.

Still, there is no need for this feature, as some kind of bool evaluation has inevitably to be made and there are many ways to reduce verbosity here short of changing the language.