Structures | Signatures | Identifiers

Signature BOOL

Basic operations on boolean values.

structure Bool : BOOL

signature BOOL =
  sig
    eqtype bool
    val not        : bool -> bool
    val toString   : bool -> string
    val scan       : (char, 'a) StringCvt.reader -> (bool, 'a) StringCvt.reader
    val fromString : string -> bool option
  end

[type bool]

is considered a primitive type and is defined in the top-level environment. It is rebound here for consistency.

[not b]

returns the logical negation of the boolean value b.

[toString b]

returns the string representation of b, either "true" or "false".

[scan getc strm]

Scans a character source for a boolean value. Takes a character stream reader getc and a stream strm. Ignoring case and initial whitespace, the sequences "true" and "false" are converted to the corresponding boolean values. On successful scanning of a boolean value, scan returns SOME(b, rest), where b is the scanned value and rest is the remaining character stream.

[fromString s]

Scans a boolean from a string s. It returns SOME(b) for a scanned value b; otherwise it returns NONE. Equivalent to StringCvt.scanString scan.

Discussion

In addition to the not function presented here, the language defines the special operators andalso and orelse, which provide short-circuit evaluation of the AND and OR of two boolean expressions. The semantics of strict AND and OR operators, which would evaluate both expressions before applying the operator, are rarely needed and can easily be obtained using the andalso and orelse operators.