Lazy lists

Lists in Scheme and Lisp are eager. Since the procedure calling regime in these languages is "Call by value", a list argument of a procedure call is completely constructed before the procedure is called. This is fine for small lists, but it excludes practically the chaining of procedure calls with large list arguments. On the other hand such a chaining is a tremendously powerful modularization technique, as demonstrated by purely functional languages like Miranda or Haskell.

The traditional tools for the implementation of lazy evaluation consist of the two Scheme primitives delay and force (cf. the classic "Structure and Interpretation of Computer Porgrams" by Abelson and Sussman, usually abbreveated "SICP"). But there is a better method as shown by Moritz Heidkamp in his lazy-seq module, which in turn is meant to replace the stream datatype in SRFI-41. Moritz' approach is inspired by the Lisp dialect Clojure, which also motivated the beautiful macros in his clojurian module. The fundamental idea is to store the structure of a lazy list in a record type, but to realize resulting record only as much as needed. This way a large (even infinite) list can be created instantaneously without realizing it and it will be realized only if and as much as used.

This module is based on Heidkamp's implementation with one essential addition: A boolean named finite? is stored in the record type and can thus be referenced without realizing the resulting record. After all, some operations like reverse are only meaningful for finite lists, so one must know beforehand if a list is finite to avoid infinite loops.

But knowing the finiteness of a list at the moment of its creation, lazy lists can replace ordinary lists as a datatype. And ordinary list operations can be replaced by lazy list operations. This is the reason for the other difference of this module with Moritz' lazy-seq, a cosmetic difference: Lazy list operations are named with the same name as ordinary ones, only capitalized at the beginning. So accessors Car, Cdr ... are the replacements of car, cdr etc. Some operators have a different argument order, thow, so that the clojurian chaining macro ->> works well. The consistent argument order is as follows: procedure arguments appear first, lazy list arguments last. For example (At n Lst) replaces (list-ref lst n), (Drop n Lst) replaces (list-tail lst n), etc. But (List-ref Lst n) and (List-tail Lst n) is still there for convenience, but discouraged.

Storing the finiteness in the list record type has another advantage: One can check finiteness of a lazy list argument at the start of a routine. One could use assert for that, but that would mean to always do all checks provided you don't compile in unsafe mode, which makes all code unsafe. So I provided another macro instead, assume-in, which does any error-checking only if a special feature, assumptions-checked, is registerd.

lazy-lists

lazy-lists #!optional symprocedure

documentation procedure: returns a sorted list of all exported symbols with no argument or the signature of sym when called with argument.

make-lazy

Make-lazy finite? thunkprocedure

lazy constructor.

Lazy

(Lazy finite? xpr . xprs)syntax

convenience wrapper to Make-lazy constructor, avoiding a thunk.

empty lazy list

List?

List? xprprocedure

lazy version of list?

List-finite?

List-finite? xprprocedure

is xpr a finite List?

List-infinite?

List-infinite? xprprocedure

is xpr an infinite List?

List-not-null?

List-not-null? xprprocedure

is xpr a non-empty List?

Lists-one-finite?

Lists-one-finite? #!rest Lstsprocedure

is Lsts a nonempty list of Lists, at least one of it being finite?

checks if n is an admissible index for the lazy-list Lst. Deprecated, makes finite Lists eager. Moreover traverses a List twice, when used in a check

Length

Length Lstprocedure

lazy version of length, returning a fixnum or #f

Cons

Cons var Lstprocedure

lazy version of cons. Deprecated, use (Lazy finite? (cons var Lst)) instead

Rest

Rest Lstprocedure

lazy version of cdr

Cdr

Cdr Lstprocedure

lazy version of cdr

First

First Lstprocedure

lazy version of car

Car

Car Lstprocedure

lazy version of car

At

At n Lstprocedure

lazy version of list-ref with changed argument order, realizing the Lst argument upto n.

Ref

Ref n Lstprocedure

alias for At, deprecated.

Null?

Null? Lstprocedure

lazy version of null?

Zip

Zip Lst1 Lst2procedure

interleave two lazy lists, both might be infinite

Unzip

Unzip Lstprocedure

splits a lazy list in two by alternatingly putting the items of Lst into the first or the second result lazy list.

Some?

Some? ok? Lstprocedure

does some item of Lst fulfill ok?

Every?

Every? ok? Lstprocedure

does every item of Lst fulfill ok?

Fold-right*

Fold-right* op base Lst #!rest Lstsprocedure

create a lazy list of right folds changing order or List items

Fold-left*

Fold-left* op base Lst #!rest Lstsprocedure

create a lazy list of left folds

Fold-right

Fold-right op base Lst #!rest Lstsprocedure

lazy version of fold-right, one of the Lsts must be finite.

Fold-left

Fold-left op base Lst #!rest Lstsprocedure

lazy version of fold-left, one of the Lsts must be finite.

Sieve

Sieve =? Lstprocedure

sieve of Erathostenes with respect to =?

Split-with

Split-with ok? Lstprocedure

split a finite lazy list at first index not fulfilling ok?

Split-at

Split-at n Lstprocedure

split a List at fixed position

List->vector

List->vector Lstprocedure

transform a finite lazy list into a vector

vector->List

vector->List vecprocedure

transform a vector into a finite lazy list

Sort

Sort <? Lstprocedure

sort a finite lazy list with respect to <?

Merge

Merge <? Lst1 Lst2procedure

merge two sorted finite lazy lists with respect to <?

Sorted?

Sorted? <? Lstprocedure

is the finite lazy lst sorted with respect to <?

Cycle

(Cycle [n] Lst)procedure

create finite List of Length n or infinite List by cycling finite List Lst

Reverse*

Reverse* Lstprocedure

List of successive reversed subLists

Reverse

Reverse Lstprocedure

lazy version of reverse. Lst must be finite

Append

Append #!rest Lstsprocedure

lazy version of append. If one of the Lsts is infinite, it's the last one to be appended.

Range

(Range [from] upto [step])procedure

List of integers from (included) upto (excluded) with step

Iterate

Iterate fn x #!optional timesprocedure

create finite List of Length times or infinite List by applying fn succesively to x

Repeatedly

Repeatedly thunk #!optional timesprocedure

create finite List of Length times or infinite List of return values of thunk

Repeat

Repeat x #!optional timesprocedure

create finite List of Length times or infinite List of x

input->List

transform input port into List with read-proc

For-each

For-each proc #!rest Lstsprocedure

lazy version of for-each. At least one of the Lsts must be finite. For-each terminates at its length.

Filter

Filter ok? Lstprocedure

lazy version of filter

Remp

Remp ok? Lstprocedure

removes all items which pass the ok? predicate.

Remove

Remove item Lstprocedure

removes all items form Lst which are equal? to item.

Remq

Remq item Lstprocedure

removes all items form Lst which are eq? to item.

Remv

Remv item Lstprocedure

removes all items form Lst which are eqv? to item.

Map

Map proc #!rest Lstsprocedure

lazy version of map, terminates at the shortest Length.

Assoc

Assoc key aLstprocedure

lazy version of assoq

Assv

Assv key aLstprocedure

lazy version of assv

Assq

Assq key aLstprocedure

lazy version of assq

Assp

Assp ok? aLstprocedure

return #f or first pair, whose Car fulfills ok?

Equal?

Equal? Lst1 Lst2procedure

lazy version of equal?

Eqv?

Eqv? Lst1 Lst2procedure

lazy version of eqv?

Eq?

Eq? Lst1 Lst2procedure

lazy version of eq?

Equ?

Equ? =? Lst1 Lst2procedure

compare two Lists with predicate =?

Member

Member var Lstprocedure

lazy version of member

Memv

Memv var Lstprocedure

lazy version of memv

Memq

Memq var Lstprocedure

lazy version of memq

Memp

Memp ok? Lstprocedure

Tail of items not fulfilling ok?

Count-while

Count-while ok? Lstprocedure

return index of first item not fulfilling ok?

Drop-while

Drop-while ok? Lstprocedure

Tail of items not fulfilling ok? Lst must be finite.

Take-while

Take-while ok? Lstprocedure

List of items fulfilling ok? Lst must be finite.

Drop

Drop n Lstprocedure

lazy version of list-tail with changed argument order

Take

Take n Lstprocedure

List of first n items of Lst

List

List #!rest argsprocedure

lazy version of list. Constructs a finite List.

list->List

list->List lstprocedure

transform ordinary list into finite lazy list

List->list

List->list Lstprocedure

transform finite lazy into ordinary list

Realize

Realize Lstprocedure

realize a finite List

Realized?

Realized? Lstprocedure

Is Lst realized?

Primes

Primesprocedure

lazy list of prime numbers

Cardinals

Cardinalsprocedure

lazy list of non negative integers

assume-in

(assume-in sym test . tests)syntax

Checks if all the assumptions test ... in the routine with name sym are valid and provides a meaningful error-message otherwise, provided the feature assumptions-checked is registered. Checks nothing if the feature is not registered.

Usage

```(require-library lazy-lists)
(import lazy-lists)```

Examples

```(require-library lazy-lists)
(import lazy-lists)

(define (cons-right var lst)
(if (null? lst)
(cons var lst)
(cons (car lst) (cons-right var (cdr lst)))))

(define (Within eps Lst)
(let loop ((Lst Lst))
(let ((a (At 0 Lst)) (b (At 1 Lst)))
(if (< (abs (- a b)) eps)
b
(loop (Rest Lst))))))

(define (Relative eps Lst)
(let loop ((Lst Lst))
(let ((a (At 0 Lst)) (b (At 1 Lst)))
(if (<= (abs (/ a b)) (* (abs b) eps))
b
(loop (Rest Lst))))))

(define (Newton x) ; fixed point for square root
(lambda (a) (/ (+ a (/ x a)) 2)))

(define (Sums Lst) ; List of sums
(let loop ((n 1))
(Lazy #f (cons (apply + (List->list (Take n Lst)))
(loop (fx+ n 1))))))

(define (First-five) (List 0 1 2 3 4))
(define (Fibs)
(Append (List 0 1) (Lazy #f (Map + (Rest (Fibs)) (Fibs)))))

;; some tests
(Length (First-five)) ;-> 5
(Length (Rest (First-five))) ;-> 4
(Length (Rest (Cardinals))) ;-> #f
(Length (Take 5 (Cardinals))) ;-> 5
(Length (Cardinals)) ;-> #f
(Length (Drop 5 (Cardinals))) ;-> #f
(First (Drop 5 (Cardinals))) ;-> 5
(List->list (First-five)) ;-> '(0 1 2 3 4)
(List->list (Take 5 (Cardinals))) ;-> '(0 1 2 3 4)
(Length (Range 2 10)) ;-> (- 10 2)
(List->list (Range 2 10)) ;-> '(2 3 4 5 6 7 8 9)
(List->list (Range 10 2)) ;-> '(10 9 8 7 6 5 4 3)
(cons (First tail) (List->list head))) ;-> '(3 0 1 2)
(receive (head index tail) (Split-with (cut < <> 5) (Take 10 (Cardinals)))
(append (List->list tail) (List->list head))) ;-> '(5 6 7 8 9 0 1 2 3 4)
(Index (cut = <> 2) (First-five)) ;-> 2
(Index (cut = <> 20) (First-five)) ;-> 5
(List->list (Take-while (cut < <> 5) (Take 10 (Cardinals))))
;-> '(0 1 2 3 4)
(Length (Take-while (cut < <> 5) (Take 10 (Cardinals)))) ;-> 5
(Length (Drop-while (cut < <> 5) (Take 10 (Cardinals)))) ;-> 5
(First (Drop-while (cut < <> 5) (Take 10 (Cardinals)))) ;-> 5
(Length (Drop-while (cut < <> 2) (First-five))) ;-> 3
(First (Drop-while (cut < <> 2) (First-five))) ;-> 2
(List->list (Memp odd? (First-five))) ;-> '(1 2 3 4)
(List->list (Memv 5 (Take 10 (Cardinals)))) ;-> '(5 6 7 8 9)
(Assv 5 (Take 10 (Map (lambda (x) (list x x)) (Cardinals))))
;-> '(5 5)
(Assv 10 (Map (lambda (x) (list x x)) (First-five))) ;-> #f
(Equal? (Cardinals) (Cardinals)) ;-> #f
(Equal? (Cardinals) (First-five)) ;-> #f
(Equal? (First-five) (First-five)) ;-> #t
(Length (Take 10 (Cardinals))) ;-> 10
(List->list (Take 5 (Filter odd? (Drop 1 (Cardinals)))))
;-> '(1 3 5 7 9)
(Length (Cardinals)) ;-> #f
(List->list (Map add1 (First-five))) ;-> '(1 2 3 4 5)
(List->list (Map + (First-five) (Take 5 (Cardinals))))
;-> '(0 2 4 6 8)
(Length (Map + (Cardinals) (Cardinals))) ;-> #f
(Length (Filter odd? (First-five))) ;-> 2
(List->list (Filter odd? (First-five))) ;-> '(1 3)
(Length (Filter odd? (Cardinals))) ;-> #f
(At 20 (Sieve = (Zip (Cardinals) (Cardinals)))) ;-> 20
(List->list (Sieve = (Zip (First-five) (First-five))))
;-> '(0 1 2 3 4)
(At 25 (Cardinals)) ;-> 25
(At 2 (First-five)) ;-> 2
(List->list (Take 3 (Repeat #f))) ;-> '(#f #f #f)
(List->list (Take 3 (Repeatedly (lambda () 1))))
;-> '(1 1 1)
(List->list (Take 3 (Iterate add1 0))) ;-> '(0 1 2)
(Length (Iterate add1 0)) ;-> #f
(Length (Append (First-five) (First-five))) ;-> 10
(List->list  (Append (First-five) (First-five)))
;-> '(0 1 2 3 4 0 1 2 3 4)
(List->list (Take 12 (Append (First-five) (Cardinals))))
; -> '(0 1 2 3 4 0 1 2 3 4 5 6)
(Length (Append (First-five) (Cardinals))) ; -> #f
(List->list (Reverse (First-five))) ; -> '(4 3 2 1 0)
(List->list (Reverse (Take 5 (Cardinals)))) ; -> '(4 3 2 1 0)
(Length (Reverse (First-five))) ; -> 5
(Length (Reverse* (Cardinals))) ; -> #f
(List->list (At 5 (Reverse* (Cardinals)))) ; -> '(5 4 3 2 1 0)
(List->list (Take 10 (Cycle (First-five))))
; -> '(0 1 2 3 4 0 1 2 3 4)
(Length (Cycle (First-five))) ; -> #f
(List->list (Sort < (First-five))) ; -> '(0 1 2 3 4)
(Length (Sort < (First-five))) ; -> 5
(List->list (Sort < (List 3 1 0 2 4))) ; -> '(0 1 2 3 4)
; -> '(5 0 1 2 3 4)
; -> '(0 1 2 3 4)
(Fold-left + 0 (Take 5 (Cardinals))) ; -> 10
(Fold-left + 0 (First-five) (First-five)) ; -> 20
(Fold-left * 1 (List 1 2 3 4)) ; -> 24
(Fold-left cons '() (Take 5 (Cardinals)))
; -> '(((((() . 0) . 1) . 2) . 3) . 4)
(At 4 (Fold-left* cons '() (Cardinals)))
; -> '(((((() . 0) . 1) . 2) . 3) . 4)
(Fold-right + 0 (Take 5 (Cardinals))) ; -> 10
(Fold-right + 0 (First-five) (First-five)) ; -> 20
;; list
(Fold-right cons '() (First-five))
; -> '(0 1 2 3 4)
;; append
(Fold-right cons '(a b c) (First-five))
; -> '(0 1 2 3 4 a b c)
(At 4 (Fold-right* cons '() (Cardinals)))
; -> '(4 3 2 1 0)) ; note changed order
(At 4 (Fold-right* cons-right '() (Cardinals)))
; -> '(0 1 2 3 4)
(At 4 (Fold-right* cons '(a b c) (Cardinals)))
; -> '(4 3 2 1 0 a b c) ; note changed order
(At 4 (Fold-right* cons-right '(a b c) (Cardinals)))
; -> '(a b c 0 1 2 3 4)
(List->list (vector->List '#(0 1 2 3 4))) ; -> '(0 1 2 3 4)
(Null? (vector->List '#())) ; -> #t
(List->vector (Take 5 (Cardinals))) ; -> '#(0 1 2 3 4)
(List->vector (First-five)) ; -> '#(0 1 2 3 4)
(List->vector Nil) ; -> '#()
(Every? odd? (Take 15 (Filter odd? (Cardinals)))) ; -> #t
(Every? odd? (Take 15 (Cardinals))) ; -> #f
(Every? odd? Nil) ; -> #t
(Some? odd? Nil) ; -> #f
(Some? odd? (Take 5 (Filter even? (Cardinals)))) ; -> #f
(Some? odd? (First-five)) ; -> #t
(Length (Zip (Cardinals) (First-five))) ; -> #f
(Length (Zip (First-five) (Cardinals))) ; -> #f
(Length (Zip (Cardinals) (Cardinals))) ; -> #f
(Length (Zip (First-five) (First-five))) ; -> 10
(Eqv? (Take 14 (Zip (Cardinals) (First-five)))
(List 0 0 1 1 2 2 3 3 4 4 5 6 7 8)) ; -> #t
(Eqv? (Take 14 (Zip (Cardinals) (Cardinals)))
(List 0 0 1 1 2 2 3 3 4 4 5 5 6 6)) ; -> #t
(Eqv? (Zip (First-five) (First-five))
(List 0 0 1 1 2 2 3 3 4 4)) ; -> #t
(At 50 (Primes)) ; -> 233
(Eqv? (Take 5 (Primes)) (List 2 3 5 7 11)) ; -> #t
(Eqv? (Take 10 (Fibs)) (List  0 1 1 2 3 5 8 13 21 34)) ; -> #t
;; compute square root
(let ((eps 0.000001))
(< (abs (- (sqrt 2) (Within eps (Iterate (Newton 2) 2)))) eps)) ; -> #t
(List->list (Sums (Take 5 (Cardinals)))) ; -> '(0 1 3 6 10)
```

Juergen Lorenz

Aug 1, 2012

Updated

Mar 03, 2017

```Copyright (c) 2012-2017, Juergen Lorenz

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Version History

0.9
replaced length slot with finite? slot to keep lazyness of finite lists
0.8.1
fixed bug in documentation procedure lazy-lists
0.8
new procedures lazy-lists Unzip Remp Remove Remq Remv
0.7
assumptions checked with assume-in
0.6
dependency on methods removed
0.5.2
tests updated
0.5.1
tests updated
0.5
Repeat(edly), Iterate, Cycle with an optinal arg, Split-with semantics changed, ...-upto changed to ...-while
0.4
List-finite? corrected, List-infinite? and Realize added
0.3
dependency changed from contracts to multi-methods, additional predicates introduced
0.2
dependency on clojurian removed
0.1
initial import