Provided by: tcllib_1.21+dfsg-1_all 
NAME
generator - Procedures for creating and using generators.
SYNOPSIS
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
_________________________________________________________________________________________________
DESCRIPTION
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 }
}
COMMANDS
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]] = $gen
PRELUDE
The following commands are provided as a standard library of generator combinators and
functions that perform convenience operations on generators. The functions in this section
are loosely modelled on the equivalent functions from the Haskell Prelude. Warning: most
of the functions in this prelude destroy any generator arguments they are passed as a
side-effect. If you want to have persistent generators, see the streams library.
generator map function generator
Apply a function to every element of a generator, returning a new generator of the
results. This is the classic map function from functional programming, applied to
generators. For example, we can generate all the square numbers using the following
code (where nats is defined as earlier):
proc square x { expr {$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.
BUGS, IDEAS, FEEDBACK
Please report any errors in this document, or in the package it describes, to Neil Madden
[mailto:nem@cs.nott.ac.uk].
KEYWORDS
control structure, coroutine, filter, foldl, foldr, foreach, generator, iterator, map,
reduce, scanl