It would be amazing if you could implement a function that allows you to set a temporary variable to use within a formula! Something like the syntax used in OpenRefine's "with()" control (borrowing their syntax for my example - link to their wiki below).

Syntax:
WITH( [Function A], [Variable Name: string], [Function B] )

[Function A] would get evaluated first, and then the result could be referenced within [Function B] using the [Variable Name] string (essentially as if it was a named range). With this you can reuse a value in a formula multiple times without needing to repeat the function.

This would:
- eliminate (or at least reduce) the need to create helper cells / columns
- reduce the need to write long, messy, complicated formulas that repeat the same function calculations
- lower the amount of overhead when parsing formulas

Here's an example. This is using a pretty simple formula just that returns a status string output. Granted, theres a bunch of ways to do this, but just for illustration:

Current possible method 1 - repeating formula:

target cell:
=IF(A4="OUT","excluded in "&IF(ISERROR(MATCH(B4,Sheet2!$C:$C,0)),"both","list A but included in list B (!)"),IF(ISERROR(MATCH(B4,Sheet2!$C:$C,0)),"included in list A but excluded from list B (!)",""))

Current possible method 2 - helper cell:

helper cell (C4):
=ISERROR(MATCH(B4,Sheet2!$C:$C,0))
target cell:
=IF(A4="OUT","excluded in "&IF(C4,"both","list A but included in list B (!)"),IF(C4,"included in list A but excluded from list B (!)",""))

Proposed method - using "with" function:

=WITH(IF(A4="OUT","ex","in"),"var1",WITH(IF(ISERROR(MATCH(B4,Sheet2!$C:$C,0)),"ex","in"),"var2",IF(var1=var2,IF(var1="ex","excluded in both",""),var1&"cluded in list A but "&var2&"cluded in list B (!)")))

https://github.com/OpenRefine/OpenRefine/wiki/GREL-Controls#withexpression-o-variable-v-expression-e