Using LAMBDA functions

EPPlus supports LAMBDA functions from version 8.2. LAMBDA lets you define custom, reusable formulas directly in the workbook — without needing VBA or external code. The EPPlus calculation engine supports the full range of LAMBDA capabilities, including defined names, recursion, currying and eta reduction.

Lambda in defined names

A LAMBDA function can be stored as a defined name, making it reusable across the entire workbook just like a built-in function.

using var package = new ExcelPackage();
var sheet = package.Workbook.Worksheets.Add("Sheet1");

// Define a reusable division function
package.Workbook.Names.AddFormula("Divide", "LAMBDA(x, y, x/y)");

sheet.Cells["A1"].Formula = "Divide(12, 3)";
sheet.Calculate();
var result = sheet.Cells["A1"].Value; // result is 4

Recursion

A LAMBDA can call itself, which allows you to implement iterative algorithms directly in a formula. The example below defines a recursive factorial function as a defined name.

// Factorial(n) = n * Factorial(n-1), base case at n = 0
// Note: the sheet name prefix is required when a defined name calls itself recursively
ws.Names.AddFormula("Factorial",
    "LAMBDA(input, LET(n, input, IF(n = 0, 1, n * Sheet1!Factorial(n - 1))))");

ws.Cells["A1"].Formula = "Factorial(4)";
ws.Calculate();
// Result: 24

Currying

A LAMBDA can return another LAMBDA, allowing you to build up a calculation by supplying one argument at a time. This technique is known as currying.

// Each call supplies one argument and returns a new function
// until all three arguments are provided
sheet.Cells["A1"].Formula = "LAMBDA(a, LAMBDA(b, LAMBDA(c, a + b + c)))(1)(2)(3)";
sheet.Calculate();
// Result: 6

Eta Reduced Lambda

In higher-order functions like MAP, REDUCE or SCAN, you normally wrap a function in a full LAMBDA to pass it as an argument. Eta reduction is a shorthand from lambda calculus that lets you pass the function directly instead, omitting the wrapper entirely.

Excel represents this using the _xleta prefix — this is part of the OOXML specification, not an EPPlus-specific construct. EPPlus supports this syntax from version 8.2.

// Standard approach — wrap UPPER in a full LAMBDA
sheet.Cells["B1"].Formula = "MAP(A1:A3, LAMBDA(x, UPPER(x)))";

// Eta reduced — pass UPPER directly using the _xleta prefix
sheet.Cells["B2"].Formula = "MAP(A1:A3, _xleta.UPPER)";

Both formulas produce the same result. The eta reduced form is more concise and is the notation Excel itself uses internally when saving certain formulas to the file format.

See also

Formula calculation