GNU Info

(slib.info)Parameter lists

---

Next: Getopt Parameter lists Prev: Command Line Up: Programs and Arguments

---

Enter node , (file) or (file)node

Parameter lists
---------------

  `(require 'parameters)'

Arguments to procedures in scheme are distinguished from each other by
their position in the procedure call.  This can be confusing when a
procedure takes many arguments, many of which are not often used.

A "parameter-list" is a way of passing named information to a
procedure.  Procedures are also defined to set unused parameters to
default values, check parameters, and combine parameter lists.

A PARAMETER has the form `(parameter-name value1 ...)'.  This format
allows for more than one value per parameter-name.

A PARAMETER-LIST is a list of PARAMETERs, each with a different
PARAMETER-NAME.

 - Function: make-parameter-list parameter-names
     Returns an empty parameter-list with slots for PARAMETER-NAMES.

 - Function: parameter-list-ref parameter-list parameter-name
     PARAMETER-NAME must name a valid slot of PARAMETER-LIST.
     `parameter-list-ref' returns the value of parameter PARAMETER-NAME
     of PARAMETER-LIST.

 - Function: remove-parameter parameter-name parameter-list
     Removes the parameter PARAMETER-NAME from PARAMETER-LIST.
     `remove-parameter' does not alter the argument PARAMETER-LIST.

     If there are more than one PARAMETER-NAME parameters, an error is
     signaled.

 - Procedure: adjoin-parameters! parameter-list parameter1 ...
     Returns PARAMETER-LIST with PARAMETER1 ... merged in.

 - Procedure: parameter-list-expand expanders parameter-list
     EXPANDERS is a list of procedures whose order matches the order of
     the PARAMETER-NAMEs in the call to `make-parameter-list' which
     created PARAMETER-LIST.  For each non-false element of EXPANDERS
     that procedure is mapped over the corresponding parameter value
     and the returned parameter lists are merged into PARAMETER-LIST.

     This process is repeated until PARAMETER-LIST stops growing.  The
     value returned from `parameter-list-expand' is unspecified.

 - Function: fill-empty-parameters defaulters parameter-list
     DEFAULTERS is a list of procedures whose order matches the order
     of the PARAMETER-NAMEs in the call to `make-parameter-list' which
     created PARAMETER-LIST.  `fill-empty-parameters' returns a new
     parameter-list with each empty parameter replaced with the list
     returned by calling the corresponding DEFAULTER with
     PARAMETER-LIST as its argument.

 - Function: check-parameters checks parameter-list
     CHECKS is a list of procedures whose order matches the order of
     the PARAMETER-NAMEs in the call to `make-parameter-list' which
     created PARAMETER-LIST.

     `check-parameters' returns PARAMETER-LIST if each CHECK of the
     corresponding PARAMETER-LIST returns non-false.  If some CHECK
     returns `#f' a warning is signaled.

In the following procedures ARITIES is a list of symbols.  The elements
of `arities' can be:

`single'
     Requires a single parameter.

`optional'
     A single parameter or no parameter is acceptable.

`boolean'
     A single boolean parameter or zero parameters is acceptable.

`nary'
     Any number of parameters are acceptable.

`nary1'
     One or more of parameters are acceptable.

 - Function: parameter-list->arglist positions arities parameter-list
     Returns PARAMETER-LIST converted to an argument list.  Parameters
     of ARITY type `single' and `boolean' are converted to the single
     value associated with them.  The other ARITY types are converted
     to lists of the value(s).

     POSITIONS is a list of positive integers whose order matches the
     order of the PARAMETER-NAMEs in the call to `make-parameter-list'
     which created PARAMETER-LIST.  The integers specify in which
     argument position the corresponding parameter should appear.