MicrosoftCharles_Exceller Maybe I was unclear. I was not saying you have to do anything as a user. I was just suggesting using the hook into the existing internal mechanism for letting Excel know what functions exist, and information about them (as displayed in the Function Wizard). This could happen behind the scenes without the user being aware.
I am not sure Microsoft had the lofty ambitions for LAMBDA that you imply. The very naming of the feature (lifting from development languages where a lambda is a small snippet of code) suggests that MS is not intending a complete new functional framework. To me, LET flowed logically from the new ability to manipulate arrays: it allows you to do a calculation once rather than for each cell in the array, and store intermediate results (eg calculate the residual for each y, in an x-y array: you only want to do the linear regression once). Once you had LET, then it followed that a mechanism to re-use LET formulas might be useful.
I do feel that from a developer perspective you could fall into the trap of "spec creep" where nothing is ever delivered as you are always trying to include more functionality (and I have worked on those projects!). In my opinion it would be better to round off the current LAMBDA development stage so that a LAMBDA looks to the user like any other spreadsheet function. The user may not care if it is in the Name Manager, a VBA module, or divined from the alignment of the planets: they just want to pass parameters and get a result.
From the security aspect, LAMBDAs can only call registered built-in or user-defined functions (as far as I know). In that sense it is VBA without access to the shell, Win32 API, COM, etc or anything that can manipulate the user's system. Maybe you could define a new type of workbook rather than .xlsm that only contains a new class of "safe" VBA functionality? The VBA editor has all the good stuff for debugging and scripting the function (eg Intellisense) so this would avoid reinventing the wheel.
