[Proposal] Add a Zip LINQ query operator

[AdamSpeight2008]

Documentation on ZIP method.
Prior Art: Roslyn-8221
Roslyn-100

From #525, separating out the discussion around a Zip operator

Dim  xs = {0, 1, 2, 4}
Dim  ys = {"A"c, "B"c, "C"c, "D"c, "E"c}
Dim  zs = From x In xs Zip y In ys
          Select New With {.x = x, .y = y)

Output

{ x = 0, y = A }
{ x = 1, y = B }
{ x = 2, y = C }
{ x = 4, y = D }

This would be lowered in to a call to Enumerable.Zip function. eg.

Dim zs = Enumerable.Zip(xs,ys, Function(x,y) (x,y) )

---

Implementation

Definition of ZipKeyword

This will be contextual keyword, expressible within a LINQ Query Clause.

<node-kind name="ZipKeyword" token-text="Zip" />

Definition of ZipClauseSyntax

<node-structure name="ZipClauseSyntax" parent="QueryClauseSyntax">
    <node-kind name ="ZipClause" />
    <child name="ZipKeyword" kind="ZipKeyword" />
    <child name="ZipWith" kind="CollectionRangeVariable" />
</node-structure>

Refactor out the parsing CollectionRangeVariable from ParseFromControlVar, so it can also be utilised by the `ParseZipClause" function,

Private Function ParseZipClause(zipKw As KeywordSyntax) As QueryClauseSyntax
    Debug.Assert(zipKw IsNot Nothing, "Expected ZIP keyword.")
    Dim rangeVar = ParseCollectionRangeVariable()
    Dim zipOp = InternalSyntaxFactory.ZipClause(zipKw, rangeVar)
     zipOp = CheckFeatureAvailability(Feature.ZipQueryExpression, zipOp)
    Return zipOp
End Function

[aarondglover]

I don't understand the proposed function of the Zip operator... Can you explain for us mere mortals?

[AdamSpeight2008]

@aarondglover updated original post.

[RevensofT]

Look great but personally I prefer to use Zip instead Select or just use Select do the zip.

Dim  zs = From x In xs, y In ys Zip (x, y)
Dim  ss = From x In xs, y In ys Select (x, y)

[AdamSpeight2008]

@RevensofT
I don't want the cross-product of the two sequences, which produces 16 items.
Instead I want to merge the two sequences in to single sequence, of 5 items.

[VBAndCs]

Great, but I can suggest a more compact syntax using tuple-alike syntax:

   Dim zs = From (x, Y) In (xs, ys)

Note that Select (x, y) is optional, since we return the exact iterator tuple. if you want to return other result:

   Dim zs = From (x, Y) In (xs, ys)
                  Select x * y

In fact I hope to generalize this tuple iterator for all loops:

For each (x, y) in (A, B)
    Console.WriteLine(x & y)  
Next