A function passed as an argument needs _xleta., so GROUPBY(…,SUM) computes #NAME? #617

What happens

The LAMBDA-helper functions can take a built-in function as an argument instead of a LAMBDA: GROUPBY(A2:A3,B2:B3,SUM), MAP(B2:B3,ABS). Excel stores that argument with a prefix of its own, _xleta.. hucre writes it bare. Even with #608 (which adds _xlfn. and _xlpm.), a file written with GROUPBY(A2:A3,B2:B3,SUM) opens without a prompt, but the cell computes #NAME?. With _xleta.SUM hand-edited in, it computes.

Expected

What Excel 16.114 for Mac saved when the bare formulas were typed in:

_xlfn.GROUPBY(A2:A3,B2:B3,_xleta.SUM)
_xlfn.PIVOTBY(A2:A3,A2:A3,B2:B3,_xleta.SUM)
_xlfn.BYROW(B2:B3,_xleta.SUM)
_xlfn.BYCOL(B2:B3,_xleta.MAX)
_xlfn.MAP(B2:B3,_xleta.ABS)
_xlfn.REDUCE(0,B2:B3,_xleta.SUM)
_xlfn.SCAN(0,B2:B3,_xleta.SUM)
_xlfn.MAKEARRAY(2,2,_xleta.PRODUCT)
_xlfn.GROUPBY(B2:B3,B2:B3,_xleta.PERCENTOF)   ← a future function gets only _xleta., no _xlfn.

Why it isn't a one-liner

_xleta. goes on a built-in function name used as a value. Telling SUM there apart from a defined name or a LAMBDA bound by LET (_xlpm.f) needs Excel's full list of built-in functions, which hucre doesn't have. The future-function list in formula-prefix.ts covers only the functions added after 2007.

Workaround

Pass a LAMBDA: GROUPBY(A2:A3,B2:B3,LAMBDA(x,SUM(x))). With #608, that's written as _xlfn.LAMBDA(_xlpm.x,SUM(_xlpm.x)), which Excel computes.

Where

src/xlsx/formula-prefix.ts (on the #608 branch until it merges). docs/PARITY.md on that branch already notes the gap.