[ref] 20 Booleans

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# 20 Booleans

### Sections

1. Fail
2. Comparisons of Booleans
3. Operations for Booleans

The two main boolean values are `true` and `false`. They stand for the logical values of the same name. They appear as values of the conditions in `if`-statements and `while`-loops. Booleans are also important as return values of filters (see Filters) such as `IsFinite` and `IsBool`. Note that it is a convention that the name of a function that returns `true` or `false` according to the outcome, starts with `Is`.

For technical reasons, also the value `fail` (see Fail) is regarded as a boolean.

- `IsBool(` obj`) C`

  tests whether obj is `true`, `false` or `fail`.

      gap> IsBool( true );  IsBool( false );  IsBool( 17 );
      true
      true
      false

  ## 20.1 Fail

- `fail V`

  The value `fail` is used to indicate situations when an operation could not be performed for the given arguments, either because of shortcomings of the arguments or because of restrictions in the implementation or computability. So for example `Position` (see Position) will return `fail` if the point searched for is not in the list.

  `fail` is simply an object that is different from every other object than itself.

  For technical reasons, `fail` is a boolean value. But note that `fail` cannot be used to form boolean expressions with `and`, `or`, and `not` (see Operations for Booleans below), and `fail` cannot appear in boolean lists (see Chapter Boolean Lists).

  ## 20.2 Comparisons of Booleans

- ``bool1`=` bool2``
- ``bool1`<>` bool2``

  The equality operator `=` evaluates to `true` if the two boolean values bool1 and bool2 are equal, i.e., both are `true` or both are `false` or both `fail`, and `false` otherwise. The inequality operator `<>` evaluates to `true` if the two boolean values bool1 and bool2 are different and `false` otherwise. This operation is also called the exclusive or, because its value is `true` if exactly one of bool1 or bool2 is `true`.

  You can compare boolean values with objects of other types. Of course they are never equal.

      gap> true = false;
      false
      gap> false = (true = fail);
      true
      gap> true <> 17;
      true

- ``bool1`<` bool2``

  The ordering of boolean values is defined by `true < false < fail`. For the comparison of booleans with other GAP objects, see Section Comparisons.

      gap> true < false;  fail >= false;
      true
      true

  ## 20.3 Operations for Booleans

  The following boolean operations are only applicable to `true` and `false`.

- ``bool1`or` bool2``

  The logical operator `or` evaluates to `true` if at least one of the two boolean operands bool1 and bool2 is `true` and to `false` otherwise.

  `or` first evaluates bool1. If the value is neither `true` nor `false` an error is signalled. If the value is `true`, then `or` returns `true` without evaluating bool2. If the value is `false`, then `or` evaluates bool2. Again, if the value is neither `true` nor `false` an error is signalled. Otherwise `or` returns the value of bool2. This short-circuited evaluation is important if the value of bool1 is `true` and evaluation of bool2 would take much time or cause an error.

  `or` is associative, i.e., it is allowed to write ``b1`or` b2`or` b3``, which is interpreted as `(`b1`or` b2`) or` b3``. `or` has the lowest precedence of the logical operators. All logical operators have lower precedence than the comparison operators `=`, `<`, `in`, etc.

      gap> true or false;
      true
      gap> false or false;
      false
      gap> i := -1;; l := [1,2,3];;
      gap> if i <= 0 or l[i] = false then     # this does not cause an error,
      >    Print("aha\n"); fi;                # because `l[i]' is not evaluated
      aha

- ``bool1`and` bool2``

  The logical operator `and` evaluates to `true` if both boolean operands bool1 and bool2 are `true` and to `false` otherwise.

  `and` first evaluates bool1. If the value is neither `true` nor `false` an error is signalled. If the value is `false`, then `and` returns `false` without evaluating bool2. If the value is `true`, then `and` evaluates bool2. Again, if the value is neither `true` nor `false` an error is signalled. Otherwise `and` returns the value of bool2. This short-circuited evaluation is important if the value of bool1 is `false` and evaluation of bool2 would take much time or cause an error.

  `and` is associative, i.e., it is allowed to write ``b1`and` b2`and` b3``, which is interpreted as `(`b1`and` b2`) and` b3``. `and` has higher precedence than the logical `or` operator, but lower than the unary logical `not` operator. All logical operators have lower precedence than the comparison operators `=`, `<`, `in`, etc.

      gap> true and false;
      false
      gap> true and true;
      true
      gap> false and 17;  # this does not cause an error, because `17' is never looked at
      false

- ``fil1`and` fil2``

  `and` can also be applied to filters. It returns a filter that when applied to some argument x, tests ``fil1`(`x`) and` fil2`(`x`)`.

      gap> andfilt:= IsPosRat and IsInt;;
      gap> andfilt( 17 );  andfilt( 1/2 );
      true
      false

- `not` bool``

  The logical operator `not` returns `true` if the boolean value bool is `false` and `true` otherwise. An error is signalled if bool does not evaluate to `true` or `false`.

  `not` has higher precedence than the other logical operators, `or` and `and`. All logical operators have lower precedence than the comparison operators `=`, `<`, `in`, etc.

      gap> true and false;
      false
      gap> not true;
      false
      gap> not false;
      true

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  GAP 4 manual
  May 2002