#unreachable

Marks a code path as logically impossible and throws if execution reaches it.

return unreachable('caller pre-validated the input')

#Usage

#unreachable(message?)

Use unreachable as a runtime assertion for invariants the type system cannot express. Its never return type allows it in any expression position. Reaching the call indicates a bug in the program or invalid data crossing a boundary.

import { unreachable } from 'massaman/control'

function parseDigit(input: string): number {
  const parsed = Number.parseInt(input, 10)

  if (Number.isNaN(parsed)) {
    return unreachable('caller pre-validated the input')
  }

  return parsed
}

#Parameters

#Returns

never returns; the function always throws.

never

#Throws

An Error with entered unreachable code, optionally followed by message.

#See also

• todo — unfinished behavior
• unimplemented — intentionally unsupported behavior