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6.4.16 SRFI-26 - specializing parameters

This SRFI provides a syntax for conveniently specializing selected parameters of a function. It can be used with,

     (use-modules (srfi srfi-26))
— library syntax: cut slot ...
— library syntax: cute slot ...

Return a new procedure which will make a call (slot ...) but with selected parameters specialized to given expressions.

An example will illustrate the idea. The following is a specialization of write, sending output to my-output-port,

          (cut write <> my-output-port)
          (lambda (obj) (write obj my-output-port))

The special symbol <> indicates a slot to be filled by an argument to the new procedure. my-output-port on the other hand is an expression to be evaluated and passed, ie. it specializes the behaviour of write.

A slot to be filled by an argument from the created procedure. Arguments are assigned to <> slots in the order they appear in the cut form, there's no way to re-arrange arguments.

The first argument to cut is usually a procedure (or expression giving a procedure), but <> is allowed there too. For example,

               (cut <> 1 2 3)
               (lambda (proc) (proc 1 2 3))

A slot to be filled by all remaining arguments from the new procedure. This can only occur at the end of a cut form.

For example, a procedure taking a variable number of arguments like max but in addition enforcing a lower bound,

               (define my-lower-bound 123)
               (cut max my-lower-bound <...>)
               (lambda arglist (apply max my-lower-bound arglist))

For cut the specializing expressions are evaluated each time the new procedure is called. For cute they're evaluated just once, when the new procedure is created. The name cute stands for “cut with evaluated arguments”. In all cases the evaluations take place in an unspecified order.

The following illustrates the difference between cut and cute,

          (cut format <> "the time is ~s" (current-time))
          (lambda (port) (format port "the time is ~s" (current-time)))
          (cute format <> "the time is ~s" (current-time))
          (let ((val (current-time)))
            (lambda (port) (format port "the time is ~s" val))

(There's no provision for a mixture of cut and cute where some expressions would be evaluated every time but others evaluated only once.)

cut is really just a shorthand for the sort of lambda forms shown in the above examples. But notice cut avoids the need to name unspecialized parameters, and is more compact. Use in functional programming style or just with map, for-each or similar is typical.

          (map (cut * 2 <>) '(1 2 3 4))
          (for-each (cut write <> my-port) my-list)