Add a let function to introduce local scope

[Joblidoboasd]

Consider the case where you need to set some non-behaviour property as follows:

{
    Foo: Lookup(ListA, title=Dropdown.Selected.Value).fooColumn,
    Bar: Lookup(ListA, title=Dropdown.Selected.Value).barColumn,
    Baz: Lookup(ListA, title=Dropdown.Selected.Value).bazColumn
}

In that example, we had to perform 3 lookups to the same item, because we have no way of storing data in properties. We cant use Set() in non-behaviour properties.

To improve this situation, why not introduce a let style operator, similar to clojure's (let) function, that lets us temporarily store values. 

With let, we could re-write the above as follows:

Let(
  {LookupItem: Lookup(ListA, title=Dropdown.Selected.Value)},
  {
      Foo: LookupItem.fooColumn,
      Bar: LookupItem.barColumn,
      Baz: LookupItem.bazColumn
  }
)

This would save two network calls!

[mdevaney]

I feel that rather than create a new function this suggestion could be used to improve upon the existing WITH function.

[Joblidoboasd]

Huh. Turns out I've pretty much described with... Maybe update the Set and UpdateContext functions to indicate that while they can't be used in non-behaviour properties, With can. 

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[ocdc2008]

@Joblidoboasd 

Maybe I'm not understanding the issue, but it seems that a Switch statement would work here. Use Set() or UpdateContext({}) to determine which case to follow, then execute a single lookup rather than all three.