dynamic-bindings.texinfo (3189B)
1 @node Dynamic bindings 2 @subsection Dynamic bindings 3 4 The dynamic extent of a procedure call is the time between when it is initiated and when it 5 returns. In Scheme, call-with-current-continuation (section 6.10) allows reentering a 6 dynamic extent after its procedure call has returned. Thus, the dynamic extent of a call 7 might not be a single, continuous time period. 8 9 This sections introduces parameter objects, which can be bound to new values for the 10 duration of a dynamic extent. The set of all parameter bindings at a given time is called 11 the dynamic environment. 12 13 @deffn procedure make-parameter init 14 @deffnx procedure make-parameter init converter 15 16 Returns a newly allocated parameter object, which is a procedure that accepts zero 17 arguments and returns the value associated with the parameter object. Initially, this value 18 is the value of @code{(@var{converter} @var{init})}, or of 19 @var{init} if the conversion procedure 20 @var{converter} is not specified. The associated value can be temporarily changed using 21 parameterize, which is described below. 22 23 The effect of passing arguments to a parameter object is implementation-dependent. 24 @end deffn 25 26 @deffn syntax parameterize (@svar{param@sub{1}} @svar{value@sub{1}})@dots{} @svar{body} 27 28 Syntax: Both @svar{param@sub{1}} and @svar{value@sub{1}} are expressions. 29 30 It is an error if the value of any @svar{param} expression is not a parameter object. 31 32 Semantics: 33 A @code{parameterize} expression is used to change the values returned by 34 specified parameter objects during the evaluation of the body. 35 36 The @svar{param} and @svar{value} expressions 37 are evaluated in an unspecified order. The @svar{body} is 38 evaluated in a dynamic environment in which calls to the 39 parameters return the results of passing the corresponding values 40 to the conversion procedure specified when the parameters were created. 41 Then the previous values of the parameters are restored without passing 42 them to the conversion procedure. 43 The results of the last 44 expression in the @svar{body} are returned as the results of the entire 45 @code{parameterize} expression. 46 47 Note: If the conversion procedure is not idempotent, the results of 48 @code{(parameterize ((x (x))) @dots{})}, 49 which appears to bind the parameter @var{x} to its current value, 50 might not be what the user expects. 51 52 If an implementation supports multiple threads of execution, then 53 @code{parameterize} must not change the associated values of any parameters 54 in any thread other than the current thread and threads created 55 inside @svar{body}. 56 57 Parameter objects can be used to specify configurable settings for a 58 computation without the need to pass the value to every 59 procedure in the call chain explicitly. 60 61 @lisp 62 (define radix 63 (make-parameter 64 10 65 (lambda (x) 66 (if (and (exact-integer? x) (<= 2 x 16)) 67 x 68 (error "invalid radix"))))) 69 70 (define (f n) (number->string n (radix))) 71 72 (f 12) @result{} "12" 73 (parameterize ((radix 2)) 74 (f 12)) @result{} "1100" 75 (f 12) @result{} "12" 76 77 (radix 16) @result{} @r{unspecified} 78 79 (parameterize ((radix 0)) 80 (f 12)) @result{} @r{error} 81 @end lisp 82 83 @end deffn