left

Builds an EitherLeft by associating mandatory business information (literal string) with a value representing the error. This is the fundamental brick to signal a contextualized failure.

Interactive example

Syntax

function left<
	GenericInformation extends string, 
	const GenericInput extends unknown = undefined
>(
  information: GenericInformation,
  input?: GenericInput
): EitherLeft<GenericInformation, GenericInput>

Parameters

• information: Literal string specifying the type of error (e.g., "user.notFound").
• input: Optional payload carrying useful data to diagnose the issue.

Return value

An EitherLeft<Information, Value> that you can filter with E.isLeft, E.hasInformation, or E.whenHasInformation.

Best practices

• Use explicit strings ("auth.invalidToken", "order.paymentFailed").
• Reuse the info as a pattern-matching key to trigger the right application response.
• Always return a homogeneous Either (all branches of a function must return the same Either type).

See also

• right – Success-side counterpart.
• error – Specialized alias based on left("error", value).
• fail – Left variant without payload.