Formula scaffolding is the practice of generating ready-to-edit Excel formulas (also called skeletons, templates, or boilerplates) that provide a working base structure you can extend, test, and debug.

Why? Many advanced Excel formulas fail because of tiny syntax mistakes like missing commas, quotes, or mismatched parentheses. Time lost due to debugging issues can be huge! Scaffolding functions are designed to eliminate that friction by generating syntax-correct, executable formula templates that you can edit step-by-step with confidence.

When? This page and the initial set of formula skeletons have come from my efforts to get faster at writing complex LET, SWITCH, REDUCE, and custom LAMBDA functions quickly and reliability in time-constrained and high-pressure situations (Excel Esports). The goal is not runtime optimization or industry standardization, but faster, more dependable formula authoring in a time crunch.

The scaffolding concept or formulas-to-generate-formulas is not new. The LAMBDIFY function by Diarmuid Early is a well-known example of a formula designed to help you write a formula. There are many levels of usefulness for skeletons and scaffolds, from basic copy/paste code snippets, to formulas that output syntax help, to formulas that create lambdas.

TIP: You can use OA Robot to save formula skeletons that you use frequently. Just create a formula robot command with the skeleton as the formula code.

On This Page

• skSWITCH(array,[var_name],[default_expr]) - Generate a LET-SWITCH formula with the cases populated from the given array.
• Best Practices for Scaffolding - A list of design considerations for skeletons and scaffold formulas.
• LET-REDUCE-LET Skeleton - A useful copy/paste skeleton for a REDUCE formula.
• Syntax Skeleton - Example of returning a skeleton as default lambda output.

skSWITCH - Generates a LET-SWITCH formula skeleton

One formula that always slows me down is SWITCH. All those quotes, and characters, and commas, and then of course making sure you've convered all the cases, and then the debugging!

To start with, here is a boilerplate code skeleton, something that can be copy/pasted.

LET(swvar,100,input2,200,input3,300,
  sw,IFERROR(SWITCH(swvar,
    "←",111,
    "↓",222,
    "→",333,
    "↑",444,
    "","BLANK",
    "UNHANDLED"
  ),"SWERROR"),
  res,sw,
  debug,HSTACK(swvar,sw),
  res
)

Keyboard AND Mouse-Efficient Naming - Almost everything about this code snippet is intentionally optimized for speed. The variables and values need to be easy mouse targets (wider than just one character), so I'm using 111 instead of 1, input2 instead of i. Avoiding "_" and " " within the code is also helpful for minimizing key presses. For example, selecting input2 requires CTRL+SHIFT+RIGHT, while selecting input_2 would require CTRL+SHIFT+RIGHT,RIGHT,RIGHT.

"SWERROR" means "Switch Error" - it does not mean that if you see it, you should do it (wink).

To take this to the next level of usefulness, I created a lambda function called skSWITCH that lets you choose an array as input and the cases are automatically generated using UNIQUE(TOCOL(array)).

skSWITCH was designed for 4 main things:

1. Generate the LET(... SWITCH(...) ) skeleton code
2. Populate the SWITCH() formula with all of the unique values from an array
3. Make debugging a little easier with an IFERROR wrap

/** 
* skSWITCH - Convert an array of values into a SWITCH() statement skeleton.
*/
/* Inputs:
*   values     : Array of case values (e.g. A1:A10). SWITCH will include all UNIQUE.
*   [var_name] : Default = sw_var. The variable used as the first argument to SWITCH.
*   [default_expr] : Default = FALSE. Default value for SWITCH (last argument).
* Output:
*   Text string containing a multi-line LET-SWITCH formula, ready to copy/paste.
*   Copy and Paste using CTRL+SHIFT+V, then F2 to edit and press ENTER for working version.
* Notes:
*   - Text values in `values` are wrapped in double quotes.
*   - Blank cells within `values` are ignored (skeleton already includes blank).
*   - Debugging built in
*/
skSWITCH = LAMBDA(values,[var_name],[default_expr],
LET(doc,"https://www.vertex42.com/lambda/scaffolding.html",
    version,"1.0.0 - 12/15/2025",
    var_name, IF(ISOMITTED(var_name), "swvar", var_name),
    default_expr, IF(ISOMITTED(default_expr), "UNHANDLED", default_expr),
    arr, UNIQUE(TOCOL(values,1)),
    cases, "    " & IF(ISTEXT(arr),"""" & arr & """",arr) & "," &
        REPT(SEQUENCE(ROWS(arr)),3) & ",",
    body, TEXTJOIN(CHAR(10), , cases),
    default_line,
            CHAR(10) & "    " & IF(ISTEXT(default_expr),
                """" & default_expr & """",
                default_expr
            ),
    result,
        "=" &
        "LET(" & var_name &",100,input2,200,input3,300," & CHAR(10) &
        "  sw,IFERROR(SWITCH(" & var_name & "," & CHAR(10) &
        body & CHAR(10) &
        "    " & CHAR(34)&CHAR(34) & "," & CHAR(34) & "BLANK" & CHAR(34) & "," & 
        default_line & CHAR(10) &
        "  )," & CHAR(34) & "SWERROR" & CHAR(34) & ")," & CHAR(10) & 
        "  res,sw," & CHAR(10) & 
        "  debug,HSTACK(" & var_name & ",sw)," & CHAR(10) & 
        "  res" & CHAR(10) & ")",
    result
));

Best Practices for Scaffolding

Here are some attributes of a code skeleton to consider, based on the goal of speed and accuracy during formula development.

These might not be the best practices, but they are the best techniques I've tried so far.

• Be Immediately Executable - Because the generated scaffold is a valid working formula, you can enter it into Excel and test one branch at a time. This avoids the all-or-nothing debugging cycle where a formula must be perfect before anything can be tested.
• Minimize Syntax Errors - Parantheses, commas, quoting, and function structure generated correctly, eliminating common authoring mistakes.
• Expandable Structural Pattern - Beginning with LET allows easy future LET-to-LAMBDA. Placeholders are included to remind where to add other inputs, LET steps, etc.
• Support Incremental Editing - Individual cases can be completed one at a time, while unimplemented cases return safe diagnostic placeholders (such as "TODO3").
• Fail Safely and Visibly - Unimplemented cases return "TODO3" or "UNHANDLED" instead of cryptic errors.
• Build-in Debugging Hooks - Diagnostic outputs, such as intermediate values or HSTACK debug views are present from the start and easily enabled. But, without adding too much complexity to the structure.
• Safe and Useful Default Behavior - Blank values, unmatched cases, and errors handled explicitly and consistently via default expressions and IFERROR
• Easy to Edit within Formula Bar - Structure designed for editing directly within the formula bar with just enough white space to make it easily readable.
• Easy Mouse Targets - Knowing when to use the mouse vs. the keyboard is an art, but making the code easy to edit with a mouse involves making the things that you want to replace fully selectable with a double-click without fine-tuned mouse positioning.
• Keyboard-Efficient Naming - Variable names, values and placeholder text is designed to minimize keystrokes during selection and editing (avoid "_" and " " and other word boundaries).

LET-REDUCE-LET Skeleton

Here is a skeleton for using the REDUCE function.

LET(input1,100,input2,200,
prestep,300,
red,REDUCE(initacc, SEQUENCE(1), LAMBDA(acc,i,
  LET(aaa,111,
  bbb,222,
  newacc,555,
  newacc
  )
)),
final,red,
final
)

The outer LET provides the option to use LET-to-LAMBDA later, but it is unnecessary to use or edit the input1 or input2 parameters. Post-processing of the results from the REDUCE function can be done by entering other steps before "final,red".

This skeleton is set up to use SEQUENCE(n) as an iterator, like "For i = 1 to n". Using SEQUENCE(n) as the array with the default being 1 allows you to test that first step, then change 1 to 2, then 1 to n, testing the result as you go.

The output of the inner LET function would generally be either newacc, VSTACK(acc,newacc), HSTACK(acc,newacc), or perhaps something even more complicated.

An important thing to remember about REDUCE and the optional [initacc] parameter: If you leave out the initial value using REDUCE(,array,LAMBDA(acc,v, expression)), then it uses the first value in array as the first accumulator value without executing the expression for that first iteration. That can be useful or confusing.

Basic Code Skeleton as a Default Output

This idea came from Craig Hatmaker's 5g modeling techniques. However, this is just a streamlined example of using a quick syntax-based skeleton.

For this MAP_IJ function, if you call it using MAP_IJ(), then it returns the syntax as text.

MAP_IJ = LAMBDA([array],[function],
IF(ISOMITTED(array),"MAP_IJ(array, LAMBDA(v,i,j,  fun_of_v_ij  ))",
LET(doc,"https://www.vertex42.com/lambda/candidates.html",
...
)));

Take Action!

Wondering if scaffolding could speed up your Excel game? Test it out! Here are some things to try:

1. Create a SWITCH formula from scratch. Then, try using skSWITCH.
2. Find an example needing a REDUCE-LET formula and try it from scratch. Then, try it with the skeleton. (Suggestion: Bonus #4 from the "Origami" case)
3. Pay attention to complex formulas you create. Could you have saved time by using a skeleton?
4. Save your commonly used skeletons as OA Robot commands or named formulas/constants.

See Also

Create LAMBDA Functions, Candidates for the Library

Acknowledgements

Thanks to Diarmuid Early and others for their input and suggestions!

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