Next: Exceptions, Previous: Procedures and continuations, Up: Core library [Contents][Index]

Gauche has two primitive lazy evaluation mechanisms.

The first one is an explicit mechanism, defined in the Scheme
standard: You mark
an expression to be evaluated lazily by `delay`

,
and you use `force`

to make the evaluation happen when
needed. Gauche also support another primitive `lazy`

,
as defined in srfi-45, for space-efficient tail-recursive
lazy algorithms.

The second one is a lazy sequence, in which evaluation happens
implicitly. From a Scheme program, a lazy sequence just looks
as a list—you can take its `car`

and `cdr`

, and
you can apply `map`

or other list procedures on it. However,
internally, its element isn’t calculated until it is required.

• Delay force and lazy: | ||

• Lazy sequences: |

Next: Lazy sequences, Previous: Lazy evaluation, Up: Lazy evaluation [Contents][Index]

Scheme has traditionally provided an explicit delayed evaluation mechanism
using `delay`

and `force`

. After R5RS, however,
it is found that it didn’t mix well with tail-recursive
algorithms: It required unbound memory, despite that the
body of the algorithm could be expressed in iterative manner.
Srfi-45 showed that introducing another primitive syntax `lazy`

addresses the issue.
For the detailed explanation please look at the srfi-45 document.
Here we explain how to use those primitives.

- Special Form:
**delay***expression* - Special Form:
**lazy***expression* [R7RS lazy][SRFI-45] These forms creates a

*promise*that delays the evaluation of`expression`.`Expression`will be evaluated when the promise is passed to`force`

.If

`expression`itself is expected to yield a promise, you should use`lazy`

. Otherwise, you should use`delay`

. If you can think in types, the difference may be clearer.lazy : Promise a -> Promise a delay : a -> Promise a

Since we don’t have static typing, we can’t enforce this usage. The programmer has to choose appropriate one from the context. Generally,

`lazy`

appears only to surround the entire body of function that express a lazy algorithm.NB: In R7RS,

`lazy`

is called`delay-force`

, for the operation is conceptually similar to`(delay (force expr))`

(note that the type of`force`

is`Promise a -> a`

).For the real-world example of use of

`lazy`

, you may want to check the implementation of`util.stream`

(see Stream library).

- Function:
**force***promise* [R7RS lazy] If

`promise`is not a promise, it is just returned.Otherwise, if

`promise`’s value hasn’t been computed,`force`

makes`promise`’s encapsulated expression be evaluated, and returns the result.Once

`promise`’s value is computed, it is memorized in it so that subsequent`force`

on it won’t cause the computation.

- Function:
**promise?***obj* [R7RS lazy] Returns

`#t`

iff`obj`is a promise object.

Previous: Delay force and lazy, Up: Lazy evaluation [Contents][Index]

A lazy sequence is a list-like structure
whose elements are calculated lazily.
Internally we have a special type of pairs, whose `cdr`

is evaluated on demand.
However, in Scheme level, you’ll never see a distinct
“lazy-pair” type. As soon as you try to access the
lazy pair, Gauche automatically *force* the delayed
calculation, and the lazy pair turns into an ordinary pair.

It means you can pass lazy sequences to ordinary list-processing
procedures such as `car`

, `cdr`

or `map`

.

Look at the following example; `generator->lseq`

takes a procedure that generates one value at a time, and
returns a lazy sequence consists of those values.

(with-input-from-file "file" (^[] (let loop ([cs (generator->lseq read-char)] [i 0]) (match cs [() #f] [(#\c (or #\a #\d) #\r . _) i] [(c . cs) (loop cs (+ i 1))]))))

It returns the position of the first occurrence of
character sequence “car” or “cdr” in the file `file`.
Characters are read as needed, so once the sequence is
found, the rest of the files won’t be read. If we do
it eagerly, we would have to read entire file first no matter how
big it is, or to give up using the mighty `match`

macro and
to write a basic state machine that reads one character one at a time.

Other than implicit forcing, Gauche’s lazy sequences are slightly different than the typical lazy stream implementations in Scheme in the following ways:

- When you construct a lazy sequence in an iterative lazy algorithm,
only
`cdr`

side of the lazy pair is lazily evaluated; the`car`

side is evaluated immediately. On the other hand, with`stream-cons`

in`util.stream`

(see Stream library), both`car`

and`cdr`

sides won’t be evaluated until it is absolutely needed. - Gauche’s lazy sequence always evaluates
*one item ahead*. Once you get a lazy pair, its`car`

part is already calculated, even if you don’t use it. In most cases you don’t need to care, for calculating one item more is a negligible overhead. However, when you create a self-referential lazy structure, in which the earlier elements of a sequence is used to calculate the latter elements of itself, a bit of caution is needed; a valid code for fully lazy circular structure may not terminate in Gauche’s lazy sequences. We’ll show a concrete example later. This bit of eagerness is also visible when side effects are involved; for example, lazy character sequence reading from a port may read one character ahead.

Note: Srfi-127 provides a portable alternative of lazy sequence
(see Lazy sequence (srfi)). It uses
dedicated APIs (e.g. `lseq-cdr`

) to operate on lazy sequences
so that portable implementation is possible. In Gauche, we just
use our built-in lazy sequence as srfi-127 lazy sequence; if you
want your code to be portable, consider using srfi-127, but be careful
not to mix lazy sequences and ordinary lists; Gauche won’t complain,
but other Scheme implementation may choke on it.

- Function:
**generator->lseq***generator* - Function:
**generator->lseq***item … generator* Creates a lazy sequence that consists of items produced by

`generator`, which is just a procedure with zero arguments that yields an item at a time. Returning EOF marks the end of the sequence (EOF itself isn’t included in the sequence). For example,`read-char`

can work as a generator. Gauche has a set of convenient utilities to deal with generators (see Generators).In the second form, the returned lazy sequence is prepended by

`item`…. Since there’s no way to distinguish lazy pairs and ordinary pairs, you can write it as`(cons* item … (generator->lseq generator))`

, but that’s more verbose.Internally, Gauche’s lazy sequence is optimized to be built on top of generators, so this procedure is the most efficient way to build lazy sequences.

Note: Srfi-127 also has

`generator->lseq`

, which is exactly the same as this in Gauche.

- Macro:
**lcons***car cdr* Returns a lazy pair consists of

`car`and`cdr`. The expression`car`is evaluated at the call of`lcons`

, but evaluation of`cdr`is delayed.You can’t distinguish a lazy pair from an ordinary pair. If you access either its

`car`

or`cdr`

, or even you ask`pair?`

to it, its cdr part is implicitly forced and you get an ordinary pair.Unlike

`cons`

,`cdr`should be an expression that yields a (lazy or ordinary) list, including an empty list. In other words, lazy sequences can always be a null-terminated list when entirely forced; there are no “improper lazy sequences”. (Since Scheme isn’t statically typed, we can’t force the`cdr`expression to be a proper list before actually evaluating it. Currently if`cdr`expression yields non-list, we just ignore it and treat as if it yielded an empty list.)(define z (lcons (begin (print 1) 'a) (begin (print 2) '()))) ⇒ ; prints '1', since the car part is evaluated eagerly. (cdr z) ⇒ () ;; and prints '2' ;; This also prints '2', for accessing car of a lazy pair forces ;; its cdr, even the cdr part isn't used. (car (lcons 'a (begin (print 2) '()))) ⇒ a ;; So as this; asking pair? to a lazy pair causes forcing its cdr. (pair? (lcons 'a (begin (print 2) '()))) ⇒ #t ;; To clarify: This doesn't print '2', because the second lazy ;; pair never be accessed, so its cdr isn't evaluated. (pair? (lcons 'a (lcons 'b (begin (print 2) '())))) ⇒ #t

Now, let me show you a case where “one item ahead” evaluation becomes an issue. The following is an elegant definition of infinite Fibonacci sequence using self-referential lazy structure (

`lmap`

is a lazy map, defined in`gauche.lazy`

module):(use gauche.lazy) ;; for lmap (define *fibs* (lcons* 0 1 (lmap + *fibs* (cdr *fibs*)))) ;; BUGGY

Unfortunately, Gauche can’t handle it well.

(car *fibs*) ⇒ 0 (cadr *fibs*) ⇒ *** ERROR: Attempt to recursively force a lazy pair.

When we want to access the second argument (

`cadr`

) of`*fibs*`

, we take the car of the second pair, which is a lazy pair of`1`

and`(lmap ...)`

. The lazy pair is forced and its cdr part needs to be calculated. The first thing`lmap`

returns needs to see the first and second element of`*fibs*`

, but the second element of`*fibs*`

is what we’re calculating now!We can workaround this issue by avoiding accessing the immediately preceding value. Fibonacci numbers F(n) = F(n-1) + F(n-2) = 2*F(n-2) + F(n-3), so we can write our sequence as follows.

(define *fibs* (lcons* 0 1 1 (lmap (^[a b] (+ a (* b 2))) *fibs* (cdr *fibs*))))

And this works!

(take *fibs* 20) ⇒ (0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181)

Many lazy algorithms are defined in terms of fully-lazy cons at the bottom. When you port such algorithms to Gauche using

`lcons`

, keep this bit of eagerness in mind.Note also that

`lcons`

needs to create a thunk to delay the evaluation. So the algorithm to construct lazy list using`lcons`

has an overhead of making closure for each item. For performance-critical part, you want to use`generator->lseq`

whenever possible.

- Macro:
**lcons****x … tail* - Macro:
**llist****x … tail* A lazy version of

`cons*`

(see List constructors). Both`lcons*`

and`llist*`

do the same thing; both names are provided for the symmetry to`cons*`

/`list*`

.The

`tail`argument should be an expression that yields a (possibly lazy) list. It is evaluated lazily. Note that the preceding elements`x`… are evaluated eagerly. The following equivalences hold.(lcons* a) ≡ a (lcons* a b) ≡ (lcons a b) (lcons* a b ... y z) ≡ (cons* a b … (lcons y z))

- Function:
**lrange***start :optional end step* Creates a lazy sequence of numbers starting from

`start`, increasing by`step`(default 1), to the maximum value that doesn’t exceed`end`. The default of`end`is`+inf.0`

, so it creates an infinite list. (Don’t type just`(lrange 0)`

in REPL, or it won’t terminate!)If any of

`start`or`step`

is inexact, the resulting sequence has inexact numbers.(take (lrange -1) 3) ⇒ (-1 0 1) (lrange 0.0 5 0.5) ⇒ (0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5) (lrange 1/4 1 1/8) ⇒ (1/4 3/8 1/2 5/8 3/4 7/8)

- Function:
**liota***:optional (count +inf.0) (start 0) (step 1)* A lazy version of

`iota`

(see List constructors); returns a lazy sequence of`count`integers (default: positive infinity), starting from`start`(default: 0), stepping by`step`(default: 1).Just like

`iota`

, the result consists of exact numbers if and only if both`start`and`step`are exact; otherwise the result consists of inexact numbers.

- Function:
**port->char-lseq***:optional port* - Function:
**port->byte-lseq***:optional port* - Function:
**port->string-lseq***:optional port* - Function:
**port->sexp-lseq***:optional port* These are the same as the following expressions, respectively. They are provided for the convenience, since this pattern appears frequently.

(generator->lseq (cut read-char port)) (generator->lseq (cut read-byte port)) (generator->lseq (cut read-line port)) (generator->lseq (cut read port))

If

`port`is omitted, the current input port is used.Note that the lazy sequence may buffer some items, so once you make an lseq from a port, only use the resulting lseq and don’t ever read from

`port`directly.Note that the lazy sequence terminates when EOF is read from the port, but the port isn’t closed. The port should be managed in larger dynamic extent where the lazy sequence is used. You can also convert input data into various lists by the following expressions (see Input utility functions). Those procedures read the port eagerly until EOF and returns the whole data in a list, while

`lseq`

versions read the port lazily.(port->list read-char port) (port->list read-byte port) (port->string-list port) (port->sexp-list port)

Those procedures make (lazy) lists out of ports. The opposite can be done by

`open-input-char-list`

and`open-input-byte-list`

; See Virtual ports, for the details.

See also Lazy sequence utilities, for more utility procedures that creates lazy sequences.

Let’s consider calculating an infinite sequence of prime numbers.
(Note: If you need prime numbers in your application, you don’t
need to write one; just use `math.prime`

. see Prime numbers).

Just pretend we already have some prime numbers
calculated in a variable `*primes*`

, and you need
to find a prime number equal to or grater than `n`
(for simplicity, we assume `n` is an odd number).

(define (next-prime n) (let loop ([ps *primes*]) (let1 p (car ps) (cond [(> (* p p) n) n] [(zero? (modulo n p)) (next-prime (+ n 2))] [else (loop (cdr ps))]))))

This procedure loops over the list of prime numbers, and
if no prime number `p` less than or equal to `(sqrt n)`

divides `n`, we can say `n` is prime. (Actual test
is done by `(> (* p p) n)`

instead of `(> p (sqrt n))`

,
for the former is faster.)
If we find some `p` divides `n`, we try a new value
`(+ n 2)`

with `next-prime`

.

Using `next-prime`

, we can make a generator that keeps generating
prime numbers. The following procedure returns a generator
that returns primes above `last`.

(define (gen-primes-above last) (^[] (set! last (next-prime (+ last 2))) last))

Using `generator->lseq`

, we can turn the generator returned
by `gen-primes-above`

into a lazy list, which can be used
as the value of `*prime*`

. The only caveat is that we need
to have some pre-calculated prime numbers:

(define *primes* (generator->lseq 2 3 5 (gen-primes-above 5)))

Be careful not to evaluate `*primes*`

directly on REPL,
since it contains an infinite list and it’ll blow up your REPL.
You can look the first 20 prime numbers instead:

(take *primes* 20) ⇒ (2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71)

Or find what the 10000-th prime number is:

(~ *primes* 10000) ⇒ 104743

Or count how many prime numbers there are below 1000000:

(any (^[p i] (and (>= p 1000000) i)) *primes* (lrange 0)) ⇒ 78498

Note: If you’re familiar with the lazy functional approach, this example may look strange. Why do we use side-effecting generators while we can define a sequence of prime numbers in pure functional way, as follows?

(use gauche.lazy) (define (prime? n) (not (any (^p (zero? (mod n p))) (ltake-while (^k (<= (* k k) n)) *primes*)))) (define (primes-from k) (if (prime? k) (lcons k (primes-from (+ k 2))) (primes-from (+ k 2)))) (define *primes* (llist* 2 3 5 (primes-from 7)))

(The module `gauche.lazy`

provides `ltake-while`

, which
is a lazy version of `take-while`

. We don’t need lazy version
of `any`

, since it immediately stops when the predicate returns
a true value.)

The use of `lcons`

and co-recursion in `primes-from`

is a typical idiom in functional programming. It’s perfectly ok
to do so in Gauche; except that the generator version is *much* faster
(when you take first 5000 primes, generator version ran 17 times faster
than co-recursion version on the author’s machine).

It doesn’t mean you should avoid co-recursive code; if an algorithm can be expressed nicely in co-recursion, it’s perfectly ok. However, watch out the subtle semantic difference from lazy functional languages—straightforward porting may or may not work.

Previous: Delay force and lazy, Up: Lazy evaluation [Contents][Index]