LET/LAMBDA and Callables

This guide reflects current behavior in formualizer-eval tests (builtins/lambda.rs and engine/tests/let_lambda.rs).

Implemented semantics

• LET requires name/value pairs plus one final expression (>= 3 args and odd argument count).
• LAMBDA returns a callable value; a callable used as a value (not invoked) yields #CALC!.
• Invocation arity is exact (LAMBDA(n, ...) called with 0 or 2 args returns #VALUE!).
• Parameter names must be unique inside LAMBDA (#VALUE! on duplicates).
• Local names are case-insensitive in both LET and LAMBDA invocation (x and X are the same binding).
• Local LET bindings shadow workbook-defined names.

Edge cases you should rely on today

=LET(x,2,x+3)                              -> 5
=LET(inc,LAMBDA(n,n+1),inc(41))            -> 42
=LAMBDA(x,x+1)                              -> #CALC!
=LET(f,LAMBDA(x,x+1),f)                     -> #CALC!
=LET(inc,LAMBDA(n,n+1),inc(1,2))           -> #VALUE!
=LET(X,1,x+1)                               -> 2

Shadowing and capture notes

• Parameter shadowing works: =LET(n,5,f,LAMBDA(n,n+1),f(10)) evaluates to 11.
• Closures capture the environment at lambda creation time (snapshot behavior):
  • =LET(k,1,f,LAMBDA(x,x+k),k,2,f(0)) evaluates to 1, not 2.
• Referencing a symbol before it is bound in LET resolves as #NAME?.

Minimal engine parity example (Rust)

use formualizer_common::{ExcelErrorKind, LiteralValue};
use formualizer_workbook::Workbook;

let mut wb = Workbook::new();
wb.add_sheet("Sheet1")?;

wb.set_formula("Sheet1", 1, 1, "=LET(inc,LAMBDA(n,n+1),inc(41))")?;
wb.set_formula("Sheet1", 1, 2, "=LET(f,LAMBDA(x,x+1),f)")?;

assert_eq!(wb.evaluate_cell("Sheet1", 1, 1)?, LiteralValue::Number(42.0));
match wb.evaluate_cell("Sheet1", 1, 2)? {
    LiteralValue::Error(err) => assert_eq!(err.kind, ExcelErrorKind::Calc),
    other => panic!("expected #CALC!, got {other:?}"),
}

Related

• Values, Coercion, and Errors
• Dependency Graph and Recalculation
• Custom Functions (Rust, Python, JS)