Gerbil Scheme

List builder

with-list-builder

(with-list-builder (put! [peek]) body ...) -> list

  put! := function identifier that modifies internal list
  peek := optional function identifier that retrieves internal list

A more efficient Syntax sugar for the call-with-list-builder procedure, so put! and peek can be used without wrapping them in a lambda first. with-list-builder returns the internal list at the end.

Examples:

> (import :std/iter)
> (with-list-builder (put!)
    (for (n (in-iota 100 1))
      (let ((mod3 (zero? (modulo n 3)))
            (mod5 (zero? (modulo n 5))))
        (put! (cond ((and mod3 mod5) "fizzbuzz")
                    (mod3 "fizz")
                    (mod5 "buzz")
                    (else n))))))
(1 2 "fizz" 4 "buzz" "fizz" ... 97 98 "fizz" "buzz")

call-with-list-builder

(call-with-list-builder proc) -> list

  proc := procedure that takes two proc identifiers as input

Takes a procedure or lambda proc which itself takes two procedures that can have any name but are called put! and peek here:

  • put! will append its input element onto the list being built.
  • peek retrieves the elements collected so far (initially []).

Note that the list is built my mutation, so that the result of peek may be mutated by subsequent calls to poke, and you must copy it with copy-list if you want not to be affected by these mutations.

Finally, call-with-list-builder returns the constructed list.

Examples:

> (import :std/iter)
> (call-with-list-builder
    (lambda (put! peek)
      (for (x (in-range 5 10))
        (displayln (peek))
        (put! (random-integer (1+ x))))))
()           ; first call to peek, no prior put!
(5)
(5 6)
(5 6 2)
(5 6 2 8)    ; fifth explicit peek call
(5 6 2 8 6)  ; final result printed by the REPL after the call returns

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