Empty type

The standard vacuum is in Théorie of the types a type which does not comprise values.

One commonly shortens it by club-footed (of bottom standard ), the symbol (⊥) or by the approximation ASCII _|_. It is called also sometimes standard zero . It should not be confused with the type signal or the Type unit. The type signal includes/understands all the values of a system. The unit type has only one value.

One often uses the empty type in the following cases:

to announce that a function or a calculation diverges ; in other words, it does not turn over a result to the appealing one. That necessarily does not mean that the program does not finish; a function can finish without turning over to its appealing, or leaving by a means other than a normal return, for example via a Continuation.
to indicate an error; that arrives mainly in theoretical languages in which the distinctions between the errors are not important. The practical computer programming languages use a management of exceptions in the place.

In Haskell the Keyword undefined represents a calculation whose result has an empty type. To try to evaluate undefined during the execution stops the program.

See too

Source

External bond

Standard and Programming Languages by Benjamin Pierce (MIT Close 2002)

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