Booleans (MIT/GNU Scheme 12.0.90)

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### 10.1 Booleans

The boolean objects are true and false. The boolean constant true is written as ‘`#t`’, and the boolean constant false is written as ‘`#f`’.

The primary use for boolean objects is in the conditional expressions `if`, `cond`, `and`, and `or`; the behavior of these expressions is determined by whether objects are true or false. These expressions count only `#f` as false. They count everything else, including `#t`, pairs, symbols, numbers, strings, vectors, and procedures as true (but see True and False).

Programmers accustomed to other dialects of Lisp should note that Scheme distinguishes `#f` and the empty list from the symbol `nil`. Similarly, `#t` is distinguished from the symbol `t`. In fact, the boolean objects (and the empty list) are not symbols at all.

Boolean constants evaluate to themselves, so you don’t need to quote them.

    #t                                      ⇒  #t
    #f                                      ⇒  #f
    '#f                                     ⇒  #f
    t                                       error→ Unbound variable

variable: false ¶
variable: true ¶

  These variables are bound to the objects `#f` and `#t` respectively. The compiler, given the `usual-integrations` declaration, replaces references to these variables with their respective values.

  Note that the symbol `true` is not equivalent to `#t`, and the symbol `false` is not equivalent to `#f`.

standard procedure: boolean? object ¶

  Returns `#t` if object is either `#t` or `#f`; otherwise returns `#f`.

      (boolean? #f)                           ⇒  #t
      (boolean? 0)                            ⇒  #f

standard procedure: not object ¶
procedure: false? object ¶

  These procedures return `#t` if object is false; otherwise they return `#f`. In other words they invert boolean values. These two procedures have identical semantics; their names are different to give different connotations to the test.

      (not #t)                                ⇒  #f
      (not 3)                                 ⇒  #f
      (not (list 3))                          ⇒  #f
      (not #f)                                ⇒  #t

extended standard procedure: procedure boolean=? boolean1 boolean2 boolean3 … ¶

  This predicate is true iff the boolean args are either all true or all false.

  Implementation note: The standard requires this procedure’s arguments to satisfy `boolean?`, but MIT/GNU Scheme allows any object to be an argument.

procedure: boolean/and object … ¶

  This procedure returns `#t` if none of its arguments are `#f`. Otherwise it returns `#f`.

procedure: boolean/or object … ¶

  This procedure returns `#f` if all of its arguments are `#f`. Otherwise it returns `#t`.

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