generator(3tcl)

📦

version 2.0 · 3 versions

2.0 · debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking (latest)
1.20 · ubuntu jammy
1.21 · debian bookworm, ubuntu noble

, in debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 NAME 💖

generator — the whole entire toolkit for making and using generators, bestie 🫶

✨ SYNOPSIS ✨

okay so here's the whole menu, no cap:

package require Tcl 8.6 9

package require generator ?0.3?

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 🌸

so the generator package hands you a bunch of commands to define and iterate over generator expressions. and what even is a generator? it's a command that returns a sequence of values 💫 — except, and this is the plot twist, unlike a normal command that just dumps a whole list on you, a generator yields each value and then suspends, so you can come back and fetch the next ones on-demand. it's giving ✨lazy✨ in the best way.

which means generators are lowkey the move when you wanna iterate over a set of values without having to build every single answer in-memory first. iconic efficiency fr fr 💅

so like, you can use them to iterate over:

elements of a data structure
rows in the result set of a database query
decoupling producer/consumer software designs, like parsers and tokenizers 👑
implementing sophisticated custom control strategies, like backtracking search

generators reduce the need to implement custom control structures — a whole bunch of those structures can be recast as generators, which gets you both a simpler implementation and a more standardised interface 🎀. the whole generator mechanism is built on top of the Tcl 8.6 coroutine mechanism, iykyk.

🦋 the ensemble situation 🦋

the package exports a single ensemble command, generator. every bit of functionality lives as a subcommand of that one command. periodt.

the core subcommands are define, yield, and foreach 💖

define works like Tcl's proc command, but it creates a generator procedure — meaning a procedure that returns a generator when you call it.
the generator itself is a command you can call multiple times: each time, it returns the next value in the generated series. and when the series is exhausted?? the generator command returns an empty list and then destroys itself 💀 (yes, it really does that).
rather than calling a generator manually though, the package also gives you 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, it's so easy bestie 🧚

writing a generator is a simple task, much like writing a normal procedure:

use the define command to define the generator
call yield instead of return

that's it. that's the whole thing 🍓

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. so if we insert a break into the loop, then any remaining numbers in the sequence would never be generated at all. no wasted work, we love to see it 😌

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 (it would just vibe there forever, rip). but 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 }
}

infinite generator, finite loop, zero tears. 💅✨

✨ COMMANDS ✨

Okay bestie, this is where the real magic happens. The generator ensemble is giving a whole toolkit, and every subcommand is a different vibe. Let's go one by one, no skips, no cap. 💅

`generator define` `name` `params` `body`

Creates a new generator procedure. The arguments to the command are literally identical to those for proc: a name, a list of params, and a body. The parameter list format is the same as 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 (a 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 🌸

okay bestie so these next commands are the whole standard library 💖 — a lil toolbox of generator combinators and functions that do convenience operations on generators. They're loosely modelled on the equivalent functions from the Haskell Prelude, iykyk.

⚠️ WARNING, and I mean this fr fr: most of the functions in this prelude destroy any generator arguments you pass them, as a side-effect 💀. Not a vibe, just the truth. If you want persistent generators, go see the streams library.

`generator map` function generator

Applies a function to every element of a generator and returns a whole new generator of the results ✨. This is the classic map from functional programming, just pointed at generators. Example: we can generate all the square numbers with 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 💎. Returns a generator that yields only those items from the argument generator that satisfy the predicate (a 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

The classic left-fold operation, periodt. 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 at the start) and that element, then uses the result as the new accumulator value. It repeats this through the entire generator (eagerly, no chill) and then returns the final accumulator value. If we think of the function as a binary operator and the zero argument as the left identity element of that operation, then reduce is folding the operator between each successive pair of values in the generator, left-associative style. 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 — and this is the plot twist — while the reduce operation is efficient for finite generators, take care not to apply it to an infinite generator, because that 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 🐌, because the entire generator needs to be evaluated into memory (as a list) before the reduction can even 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., it 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 💀

Found an error in this document, or in the package it describes? Please report it to Neil Madden [mailto:nem@cs.nott.ac.uk]. 🫶

✨ KEYWORDS ✨

control structure, coroutine, filter, foldl, foldr, foreach, generator, iterator, map, reduce, scanl

and that's a wrap bestie 🎀 this whole thing lives in tcllib, version 0.3, and it's the generator(3tcl) page — go forth and fold, you absolute icon 👑💅

📦 where this page comes from

debian trixie: tcllib 2.0+dfsg-4 · Tcl
ubuntu questing: tcllib 2.0+dfsg-4
ubuntu resolute: tcllib 2.0+dfsg-5 · Tcl
ubuntu stonking: tcllib 2.0+dfsg-6 · Tcl
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms