whenIsLeft

Executes a function only when the input is an EitherLeft. Otherwise, the original value is returned as-is.

Interactive example

Syntax

Classic signature

function whenIsLeft<
  const GenericInput extends unknown,
  const GenericOutput extends AnyValue | EscapeVoid
>(
  input: GenericInput,
  theFunction: (value: Unwrap<Extract<GenericInput, EitherLeft>>) => GenericOutput
): Exclude<GenericInput, EitherLeft> | GenericOutput;

Curried signature

function whenIsLeft<
  const GenericInput extends unknown,
  const GenericOutput extends AnyValue | EscapeVoid
>(
  theFunction: (value: Unwrap<Extract<GenericInput, EitherLeft>>) => GenericOutput
): (input: GenericInput) => Exclude<GenericInput, EitherLeft> | GenericOutput;

Parameters

• theFunction: Callback triggered only on the Left part.
• input (classic overload): Value to process immediately. Without this argument, the function returns a curried version suited for pipelines.

Return value

• Classic signature: returns either the transformed value (if Left) or the initial value.
• Curried signature: returns a function ready to use in pipe, map, etc.

Use cases

• Log or transform an error without touching the Right.
• Convert a Left into another type (e.g., EitherLeft -> string).
• Simplify repetitive if (E.isLeft(...)) blocks in a functional style.

See also

• whenIsRight – Success-side counterpart.
• isLeft – Manual type guard.
• whenHasInformation – Pattern matching on literal information.