Acme.Undefined

This module provides an alternative implementation of "Prelude.undefined", intended exclusively for denoting truly undefinable values.

Usage

module AwesomeSauce where

import Prelude hiding (undefined)
import Acme.Omitted
import Acme.Undefined

tooLazyToDefine     = (...)

actuallyUndefinable = undefined

main = do
  merelyOmitted <- isOmitted tooLazyToDefine
  putStrLn "Definition was merely omitted"
  (...)
  trulyUndefined <- isUndefined actuallyUndefinable
  putStrLn "Definition is truly undefinable"

"Undefined" redefined

Lacking a dedicated name for omitted defintions, users of Standard Haskell are left with no choice but to use "undefined" for both the undefinable and the omitted. This makes the standard implementation of "undefined" deficient, we cannot be sure what the programmer has intended, only that the definition is missing. The following implementation of undefined is similar in every way to the standard implementation, but is free from conceptual contamination.

undefined :: a

Denotes all values that are, fundamentally, undefinable.

The contract of undefined is that it will never be used for merely omitted definitions.

Observing the difference between "omitted" and "undefined"

Consistent use of undefined and omitted can clarify the intent of the programmer, but there is still no way to statically prevent incorrect uses of undefined (e.g., due to ignorance). Consequently, isUndefined will return bogus results every now and then, which is why it is modelled as an IO action and not a pure function.

Nevertheless, the user can identify incorrect uses of undefined more easily than before. To wit, if

isUndefined twoPlusTwo = return True

then, surely, something is amiss. We know that the programmer has made the mistake of believing 2+2 to be undefined, that she has not simply run out of time or gotten an important phone call while writing down the solution.

For backwards-compatibility, we also support detecting the standard implementation of undefined, about which we cannot infer anything except that its evaluation will terminate with no useful result.

isOmitted :: a -> IO Bool

Answer the age-old question "was this definition omitted?"

 isOmitted 0           = return False
 isOmitted undefined = return False
 isOmitted omitted   = return True

isUndefined :: a -> IO Bool

... or is it really undefined?

 isUndefined 0           = return False
 isUndefined undefined = return True
 isUndefined omitted   = return False

isPreludeUndefined :: a -> IO Bool

A version of isUndefined for "Prelude.undefined".