generator - Procedures for creating and using generators.
package require Tcl 8.6
package require generator ?0.2?
generator define name params
body
generator yield arg ?args..?
generator foreach varList generator
varList generator ?...? body
generator next generator
?varName..?
generator exists generator
generator names
generator destroy ?generator..?
generator finally cmd ?arg..?
generator from format value
generator to format generator
generator map function generator
generator filter predicate
generator
generator reduce function zero
generator
generator foldl function zero
generator
generator foldr function zero
generator
generator all predicate generator
generator and generator
generator any generator
generator concat generator
?generator..?
generator concatMap function
generator
generator drop n generator
generator dropWhile predicate
generator
generator contains element
generator
generator foldl1 function
generator
generator foldli function zero
generator
generator foldri function zero
generator
generator head generator
generator tail generator
generator init generator
generator takeList n generator
generator take n generator
generator iterate function init
generator last generator
generator length generator
generator or predicate generator
generator product generator
generator repeat n value..
generator sum generator
generator takeWhile predicate
generator
generator splitWhen predicate
generator
generator scanl function zero
generator
The generator package provides commands to define and
iterate over generator expressions. A generator is a command that
returns a sequence of values. However, unlike an ordinary command that
returns a list, a generator yields each value and then suspends,
allowing subsequent values to be fetched on-demand. As such, generators can
be used to efficiently iterate over a set of values, without having to
generate all answers in-memory. Generators can be used to iterate over
elements of a data structure, or rows in the result set of a database query,
or to decouple producer/consumer software designs such as parsers and
tokenizers, or to implement sophisticated custom control strategies such as
backtracking search. Generators reduce the need to implement custom control
structures, as many such structures can be recast as generators, leading to
both a simpler implementation and a more standardised interface. The
generator mechanism is built on top of the Tcl 8.6 coroutine mechanism.
The package exports a single ensemble command, generator.
All functionality is provided as subcommands of this command. The core
subcommands of the package are define, yield, and
foreach. The define command works like Tcl's proc
command, but creates a generator procedure; that is, a procedure that
returns a generator when called. The generator itself is a command that can
be called multiple times: each time it returns the next value in the
generated series. When the series has been exhausted, the generator command
returns an empty list and then destroys itself. Rather than manually call a
generator, however, the package also provides a flexible foreach
command that loops through the values of one or more generators. This loop
construct mimicks the functionality of the built-in Tcl foreach
command, including handling multiple return values and looping over multiple
generators at once. Writing a generator is also a simple task, much like
writing a normal procedure: simply use the define command to define
the generator, and then call yield instead of return. For
example, we can define a generator for looping through the integers in a
particular range:
generator define range {n m} {
for {set i $n} {$i <= $m} {incr i} { generator yield $i }
}
generator foreach x [range 1 10] {
puts "x = $x"
}
The above example will print the numbers from 1 to 10 in sequence,
as you would expect. The difference from a normal loop over a list is that
the numbers are only generated as they are needed. If we insert a break into
the loop then any remaining numbers in the sequence would never be
generated. To illustrate, we can define a generator that produces the
sequence of natural numbers: an infinite series. A normal procedure would
never return trying to produce this series as a list. By using a generator
we only have to generate those values which are actually used:
generator define nats {} {
while 1 { generator yield [incr nat] }
}
generator foreach n [nats] {
if {$n > 100} { break }
}
- generator
define name params body
- Creates a new generator procedure. The arguments to the command are
identical to those for proc: a name, a list of parameters,
and a body. The parameter list format is identical to a procedure. In
particular, default values and the ?args? syntax can be used as usual.
Each time the resulting generator procedure is called it creates a new
generator command (coroutine) that will yield a list of values on each
call. Each result from a generator is guaranteed to be a non-empty list of
values. When a generator is exhausted it returns an empty list and then
destroys itself to free up resources. It is an error to attempt to call an
exhausted generator as the command no longer exists.
- generator
yield arg ?args..?
- Used in the definition of a generator, this command returns the next set
of values to the consumer. Once the yield command has been called
the generator will suspend to allow the consumer to process that value.
When the next value is requested, the generator will resume as if the
yield command had just returned, and can continue processing to yield the
next result. The yield command must be called with at least one
argument, but can be called with multiple arguments, in which case this is
equivalent to calling yield once for each argument.
- generator
foreach varList generator varList
generator ?...? body
- Loops through one or more generators, assigning the next values to
variables and then executing the loop body. Works much like the built-in
foreach command, but working with generators rather than lists.
Multiple generators can be iterated over in parallel, and multiple results
can be retrieved from a single generator at once. Like the built-in
foreach, the loop will continue until all of the generators have
been exhausted: variables for generators that are exhausted early will be
set to the empty string.
The foreach command will automatically clean-up all of
the generators at the end of the loop, regardless of whether the loop
terminated early or not. This behaviour is provided as a convenience to
avoid having to explicitly clean up a generator in the usual cases.
Generators can however be destroyed before the end of the loop, in which
case the loop will continue as normal until all the other generators
have been destroyed or exhausted.
The foreach command does not take a snapshot of the
generator. Any changes in the state of the generator made inside the
loop or by other code will affect the state of the loop. In particular,
if the code in the loop invokes the generator to manually retrieve the
next element, this element will then be excluded from the loop, and the
next iteration will continue from the element after that one. Care
should be taken to avoid concurrent updates to generators unless this
behaviour is required (e.g., in argument processing).
- generator
next generator ?varName..?
- Manually retrieves the next values from a generator. One value is
retrieved for each variable supplied and assigned to the corresponding
variable. If the generator becomes exhausted at any time then any
remaining variables are set to the empty string.
- generator
exists generator
- Returns 1 if the generator (still) exists, or 0 otherwise.
- generator
names
- Returns a list of all currently existing generator commands.
- generator
destroy ?generator..?
- Destroys one or more generators, freeing any associated resources.
- generator
finally cmd ?arg..?
- Used in the definition of a generator procedure, this command arranges for
a resource to be cleaned up whenever the generator is destroyed, either
explicitly or implicitly when the generator is exhausted. This command can
be used like a finally block in the try command, except that
it is tied to the life-cycle of the generator rather than to a particular
scope. For example, if we create a generator to iterate over the lines in
a text file, we can use finally to ensure that the file is closed
whenever the generator is destroyed:
generator define lines file {
set in [open $file]
# Ensure file is always closed
generator finally close $in
while {[gets $in line] >= 0} {
generator yield $line
}
}
generator foreach line [lines /etc/passwd] {
puts "[incr count]: $line"
if {$count > 10} { break }
}
# File will be closed even on early exit
If you create a generator that consumes another generator (such as
the standard map and filter generators defined later), then
you should use a finally command to ensure that this generator is
destroyed when its parent is. For example, the map generator is
defined as follows:
generator define map {f xs} {
generator finally generator destroy $xs
generator foreach x $xs { generator yield [{*}$f $x] }
}
- generator
from format value
- Creates a generator from a data structure. Currently, supported formats
are list, dict, or string. The list format yields
each element in turn. For dictionaries, each key and value are yielded
separately. Finally, strings are yielded a character at a time.
- generator
to format generator
- Converts a generator into a data structure. This is the reverse operation
of the from command, and supports the same data structures. The two
operations obey the following identity laws (where = is interpreted
appropriately):
[generator to $fmt [generator from $fmt $value]] = $value
[generator from $fmt [generator to $fmt $gen]] = $gen
The following commands are provided as a standard library of
generator combinators and functions that perform convenience operations on
generators. The functions in this section are loosely modelled on the
equivalent functions from the Haskell Prelude. Warning: most of the
functions in this prelude destroy any generator arguments they are passed as
a side-effect. If you want to have persistent generators, see the streams
library.
- generator
map function generator
- Apply a function to every element of a generator, returning a new
generator of the results. This is the classic map function from functional
programming, applied to generators. For example, we can generate all the
square numbers using the following code (where nats is defined as
earlier):
proc square x { expr {$x * $x} }
generator foreach n [generator map square [nats]] {
puts "n = $n"
if {$n > 1000} { break }
}
- generator
filter predicate generator
- Another classic functional programming gem. This command returns a
generator that yields only those items from the argument generator that
satisfy the predicate (boolean function). For example, if we had a
generator employees that returned a stream of dictionaries
representing people, we could filter all those whose salaries are above
100,000 dollars (or whichever currency you prefer) using a simple filter:
proc salary> {amount person} { expr {[dict get $person salary] > $amount} }
set fat-cats [generator filter {salary> 100000} $employees]
- generator
reduce function zero generator
- This is the classic left-fold operation. This command takes a function, an
initial value, and a generator of values. For each element in the
generator it applies the function to the current accumulator value (the
zero argument initially) and that element, and then uses the result
as the new accumulator value. This process is repeated through the entire
generator (eagerly) and the final accumulator value is then returned. If
we consider the function to be a binary operator, and the zero argument to
be the left identity element of that operation, then we can consider the
reduce command as folding the operator between each
successive pair of values in the generator in a left-associative fashion.
For example, the sum of a sequence of numbers can be calculated by folding
a + operator between them, with 0 as the identity:
# sum xs = reduce + 0 xs
# sum [range 1 5] = reduce + 0 [range 1 5]
# = reduce + [+ 0 1] [range 2 5]
# = reduce + [+ 1 2] [range 3 5]
# = ...
# = reduce + [+ 10 5] <empty>
# = ((((0+1)+2)+3)+4)+5
# = 15
proc + {a b} { expr {$a + $b} }
proc sum gen { generator reduce + 0 $gen }
puts [sum [range 1 10]]
The reduce operation is an extremely useful one, and a
great variety of different operations can be defined using it. For example,
we can define a factorial function as the product of a range using
generators. This definition is both very clear and also quite efficient (in
both memory and running time):
proc * {x y} { expr {$x * $y} }
proc prod gen { generator reduce * 0 $gen }
proc fac n { prod [range 1 $n] }
However, while the reduce operation is efficient for finite
generators, care should be taken not to apply it to an infinite generator,
as this will result in an infinite loop:
sum [nats]; # Never returns
- generator
foldl function zero generator
- This is an alias for the reduce command.
- generator
foldr function zero generator
- This is the right-associative version of reduce. This operation is
generally inefficient, as the entire generator needs to be evaluated into
memory (as a list) before the reduction can commence. In an eagerly
evaluated language like Tcl, this operation has limited use, and should be
avoided if possible.
- generator
all predicate generator
- Returns true if all elements of the generator satisfy the given
predicate.
- generator
and generator
- Returns true if all elements of the generator are true (i.e., takes the
logical conjunction of the elements).
- generator
any generator
- Returns true if any of the elements of the generator are true (i.e.,
logical disjunction).
- generator
concat generator ?generator..?
- Returns a generator which is the concatenation of each of the argument
generators.
- generator
concatMap function generator
- Given a function which maps a value to a series of values, and a generator
of values of that type, returns a generator of all of the results in one
flat series. Equivalent to concat applied to the result of
map.
- generator
drop n generator
- Removes the given number of elements from the front of the generator and
returns the resulting generator with those elements removed.
- generator
dropWhile predicate generator
- Removes all elements from the front of the generator that satisfy the
predicate.
- generator
contains element generator
- Returns true if the generator contains the given element. Note that this
will destroy the generator!
- generator
foldl1 function generator
- A version of foldl that takes the zero argument from the
first element of the generator. Therefore this function is only valid on
non-empty generators.
- generator
foldli function zero generator
- A version of foldl that supplies the integer index of each element
as the first argument to the function. The first element in the generator
at this point is given index 0.
- generator
foldri function zero generator
- Right-associative version of foldli.
- generator
head generator
- Returns the first element of the generator.
- generator
tail generator
- Removes the first element of the generator, returning the rest.
- generator
init generator
- Returns a new generator consisting of all elements except the last of the
argument generator.
- generator
takeList n generator
- Returns the next n elements of the generator as a list. If not
enough elements are left in the generator, then just the remaining
elements are returned.
- generator
take n generator
- Returns the next n elements of the generator as a new generator.
The old generator is destroyed.
- generator
iterate function init
- Returns an infinite generator formed by repeatedly applying the function
to the initial argument. For example, the Fibonacci numbers can be defined
as follows:
proc fst pair { lindex $pair 0 }
proc snd pair { lindex $pair 1 }
proc nextFib ab { list [snd $ab] [expr {[fst $ab] + [snd $ab]}] }
proc fibs {} { generator map fst [generator iterate nextFib {0 1}] }
- generator
last generator
- Returns the last element of the generator (if it exists).
- generator
length generator
- Returns the length of the generator, destroying it in the process.
- generator
or predicate generator
- Returns 1 if any of the elements of the generator satisfy the
predicate.
- generator
product generator
- Returns the product of the numbers in a generator.
- generator
repeat n value..
- Returns a generator that consists of n copies of the given
elements. The special value Inf can be used to generate an infinite
sequence.
- generator
sum generator
- Returns the sum of the values in the generator.
- generator
takeWhile predicate generator
- Returns a generator of the first elements in the argument generator that
satisfy the predicate.
- generator
splitWhen predicate generator
- Splits the generator into lists of elements using the predicate to
identify delimiters. The resulting lists are returned as a generator.
Elements matching the delimiter predicate are discarded. For example, to
split up a generator using the string "|" as a delimiter:
set xs [generator from list {a | b | c}]
generator split {string equal "|"} $xs ;# returns a then b then c
- generator
scanl function zero generator
- Similar to foldl, but returns a generator of all of the
intermediate values for the accumulator argument. The final element of
this generator is equivalent to foldl called on the same
arguments.
Please report any errors in this document, or in the package it
describes, to Neil Madden [mailto:nem@cs.nott.ac.uk].
control structure, coroutine, filter, foldl, foldr, foreach,
generator, iterator, map, reduce, scanl