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