Add JoinOn and GroupJoinOn query methods

[derino]

No description provided.

[ahmetb]

Are there .NET equivalents of these? We mostly maintain the symmetry.

[derino]

No there aren't. Though, I think they enable more concise code where the intent is more clear.

This is how ( e1 ) . JoinOn( e2, predicatefn ) could be implemented with the existing methods:

( e1 ) . SelectManyBy( x1 => e2 , ( x1 , x2 ) => new { x1 , x2 } ) . Where( predicatefn )

where the predicate function takes a pair and returns boolean.

Similarly, ( e1 ) . GroupJoinOn( e2, predicatefn, resultSelector ) is equivalent to:

( e1 ) . SelectManyBy( x1 => e2 , ( x1 , x2 ) => new { x1 , x2 } ) . Where( predicatefn ) . GroupBy( { x1, x2 } => x1, { x1, x2 } => x2 ) . Select( resultSelector )

Of course these are much more verbose when written in Go.

Maybe it would be more consistent with the rest of LINQ if they were named JoinBy and GroupJoinBy. I can change that if you think so.

If you are strict with symmetry, please don't hesitate to close the PR.

P.S.: Thanks for this package!

[ahmetb]

I think we've been largely maintaining the symmetry with .NET. Other maintainers can also chime in if they feel like this is needed.