Iter - An iteration construct for Clojure

An iteration and looping DSL (Domain Specific Langauge) for Clojure. iter provides an extensible looping language that is an alternate to higher order functions.

For example

(iter (foreach x [1 2 3 4 5 6 7])
      (when (> x 2)
        (collect (* x x))))

  => (9 16 25 36 49)

Iter is inspired to the Common Lisp Iterate package https://common-lisp.net/project/iterate/doc/index.html and has some similarities to list comprehension systems.

Installation

Add the following to your project.clj :dependencies

and then require the iter macro:

(require '[iter.core :refer [iter]])

or

(ns my.app
  (:require [iter.core :refer [iter]]))

Overview

iter is a way of iterating and looping in Clojure. It provides an alternate to using the traditional Clojure tools such as higher order functions and threading macros. iter provides a rich set of operators to walk a sequence, return a lazy sequence, return a single expression, or reduce a complex sequence.

Let's walk through a simple example:

(iter (foreach x [1 2 3 4 5])
      (collect (+ x 100)))

  => (101 102 103 104 105)

The iter macro contains a mix of iter operators and regular Clojure expressions. In this example,

foreach binds the variable x in turn to each element in the Clojure sequence [1 2 3 4 5].
The variable x is now in scope for the rest of the iter form.
The collect operator adds the result of the regular Clojure expression (+ x 100) to the lazy sequence that iter returns.
The lazy sequence (101 102 103 104 105) is returned.

Iter has a rich set of operators so you can perform more complex processing. Let's look at some more examples, the first with a simple if branch::

(iter (foreach x [3 1 4 1 5 9 2 6 5 3 5 8])
      (if (even? x)
          (collect (* x x))))

  => (16 4 36 64)

and with an if/else operator:

(iter (foreach x (range 10))
      (if (even? x)
          (collect (+ 10 x))
          (collect (int (Math/pow 2 x)))))

  => (10 2 12 8 14 32 16 128 18 512)

and with multiple collects:

(iter (foreach num (range 5))
      (collect (* num num))
      (when (even? num)
        (collect (+ 10 num))))

  => (0 10 1 4 12 9 16 14)

It's also easy to build up a hash-map:

(iter (foreach word ["apple" "peaches" "pumpkin pie"])
      (collect-map word (count word)))

  => {"pumpkin pie" 11, "peaches" 7, "apple" 5}

Or a unique sequence:

(iter (foreach x [3 1 4 1 5 9 2 6 5 3 5 8])
      (collect-uniq (+ x 10)))

  => (13 11 14 15 19 12 16 18)

You can also reduce a sequence:

(iter (foreach x (range 10))
      (when (odd? x)
        (reducing x :by +)))

  => 25

Or bind new variables within the scope of iter:

(iter (foreach x (range 10))
      (let [y (* x x)]
        (when (> y 20)
          (collect y))))

  => (25 36 49 64 81)

Or maximize a value:

(iter (foreach x [3 1 4 1 5 9 2 6 5 3 5 8])
      (maximizing x))

  => 9

(iter (foreach s (clojure.string/split
                  "Lisp is a programmable programming language" #" "))
      (maximizing s :using (count s)))

  => "programmable"

In general, the iter macro contains a series of forms, where some of the forms are iter operators and the rest of the forms are just plain Clojure expressions.

How it works

The way that iter works is by parsing the input forms, translating the iter operators and leaving the remaining Clojure forms alone. A recursive Clojure expression is generated by the macro to iterate through all the inputs. Think of the iter DSL as a state machine where each form is an action to move the loop to the next state.

The state machine generated does basically three things:

It iterates through a sequence, collection, or expression. The foreach operator is the most common way to iterate.
It collects results in a lazy sequence. The collect operator is the most common way to collect.
It accumulates results using the accum operator. All accum variables are always available to use or modify.

One important point: all the non-iter operators are left completely alone. There is no walking of Clojure forms to find the iter operators hidden inside them.

As mentioned, there are builtin iter operators like collect, foreach, and accum that can be used to create new operators with the define-iter-op clause.

Nested Loops

When you have more that one iteration operator then iter creates nested loops:

(iter (foreach row [1 2 3])
      (collect [:row row])
      (foreach col ["a" "b"])
      (collect [row col]))

  => ([:row 1] [1 "a"] [1 "b"]
      [:row 2] [2 "a"] [2 "b"]
      [:row 3] [3 "a"] [3 "b"])

This nesting behavior holds true for all the Iteration Operators (see below).

If you want to avoid nesting and process the sequences in parallel, you can use multiple sequences with the foreach operator:

(iter (foreach [row col] [1 2 3 4 5] ["a" "b" "c" "d"])
      (collect [row col]))

=> ([1 "a"] [2 "b"] [3 "c"] [4 "d"])

where the sequences are truncated to the length of the shortest sequence.

Why?

It's reasonable to ask why iter exists at all. Why not just use the right selection of threading macros, sequence processing functions, and transducers?

In more complex cases, it can be clearer to use a custom DSL. Consider these contrived examples:

(iter (foreach [k v] my-map)
      (when (> v 3)
        (collect-map (if (keyword? k) (name k) k)
                     v)))

and

(->> my-map
     (filter (fn [[k v]] (> v 3)))
     (map (fn [[k v]] [(if (keyword? k) (name k) k)
                       v]))
     (into {}))

Both of these expressions produce the same result, but the iter version is a more direct way to describe the solution.

On a more philosophical note, iter is designed to reason about one element at a time and all the steps to transform that element. Threading macros, on the other hand, are designed to reason about each step in the transformation of an entire sequence and how to connect the transformation steps.

Operators

Iteration Operators

foreach

(foreach var coll)

(foreach var coll & colls)

foreach iterates over each element in coll. coll is treated as a sequence and each element in the sequence is bound to var for each iteration. We iterate through coll by repeatedly calling rest on it unti empty? returns true.

If more than one sequence is given we iterate over the sequences simultaneously in the same way that map does.

Note that normal Clojure destructuring applies when binding var.

Examples:

(iter (foreach x (range 10))
      (prn x))

(with-open [in (clojure.java.io/reader "/etc/passwd")]
  (iter (foreach line (line-seq in))
        (when (re-find #"daemon" line)
          (return line))))

(iter (foreach [name i] [:a :b :c :d] (range))
      (collect-map name i))

See Nested Loops for examples of nested loops.

forlist

(forlist var coll)

forlist is like foreach, but instead of iterating over each element it iterates over successive sub-sequences of coll. For example,

(iter (forlist x [:a :b :c :d])
      (prn x))

when evaluated, prints

[:a :b :c :d]
(:b :c :d)
(:c :d)
(:d)

The forlist operator is useful for comparing elements further down the list. Here's an example for finding the distance between elements in a vector:

(iter (forlist nums [2 7 1 8 2 8 1 8 2])
      (let [head (first nums)
            tail (rest nums)
            dist (.indexOf tail head)]
        (when (> dist 0)
          (collect [head dist]))))

  => ([2 3] [1 3] [8 1] [2 3] [8 1])

forever

(forever)

Iterate an infinite number of times. The loop runs forever until stopped by a control flow operator.

(take 10
      (iter (forever)
            (collect :a)))

  => (:a :a :a :a :a :a :a :a :a :a)

fornext

(fornext var next-expr)

(fornext var next-expr init)

(fornext var next-expr init done?)

This is the iteration building block. var starts out bound to init and is then bound to next-expr for every loop iteration. The loop terminates when done? returns true.

If not given, init defaults to next-expr and done? defaults to false.

You usually don't need to call fornext directly, but all the Iteration Operators are implemented uing fornext.

To create an infinite loop:

(iter (fornext tmp nil nil false)
      (collect :a))

To count from 1 to 10:

(iter (fornext i (inc i) 1 (> i 10))
      (collect i))

  => (1 2 3 4 5 6 7 8 9 10)

These will work, but calling forever and range is probably clearer:

(iter (forever)
      (collect :a))

(iter (foreach i (range 1 11))
      (collect i))

times

(times n)

Iterate n times.

(iter (times 8)
      (collect (rand-int 100)))

  => (82 77 85 25 10 13 5 0)

Collection operators

collect

(collect expr)

Produce a lazy sequence of expr each time collect is called. This is the basic way to produce results.

(iter (foreach x (range 5))
      (collect (* x x x)))

  => (0 1 8 27 64)

The collect operator can be called as many or as few times as needed:

(iter (foreach x (range 5))
      (when (even? x)
        (collect "even!"))
      (collect x)
      (collect (* x x)))

  => ("even!" 0 0 1 1 "even!" 2 4 3 9 "even!" 4 16)

collect-cat

(collect-cat seq-expr)

Just like collect, but concat seq-expr onto the result.

(iter (foreach x (range 5))
      (collect-cat [x (* x x)]))

  => (0 0 1 1 2 4 3 9 4 16)

The value of seq-expr must be a sequence and not a scalar.

collect-map

(collect-map key value)

Similar to collect, but produces a hash-map instead of a sequence. This produces the actual hash-map and is not lazy.

(iter (foreach x (range 10))
      (collect-map x (* x x)))

  => {0 0, 7 49, 1 1, 4 16, 6 36, 3 9, 2 4, 9 81, 5 25, 8 64}

(iter (foreach x (range 10))
      (collect-map (clojure.pprint/cl-format nil "~r" x)
                   x))

  => {"six" 6, "three" 3, "two" 2, "seven" 7, "zero" 0,
      "five" 5, "eight" 8, "one" 1, "nine" 9, "four" 4}

collect-uniq

(collect-uniq expr)

Just like collect, but returns a unique/distinct lazy sequence of values:

(iter (foreach x [3 1 4 1 5 9 2 6 5 3 5 8])
      (collect-uniq x))

  => (3 1 4 5 9 2 6 8)

collect-freq

(collect-freq expr)

Just like collect, but returns a frequency count of the values:

(iter (foreach x [2 7 1 8 2 8 1 8])
      (when (even? x)
        (collect-freq (cl-format nil "~r" x))))

  => {"two" 2, "eight" 3}

Control Flow Operators

if

(if test then)

(if test then else)

Just like the normal Clojure if, but modified to support iter. That is, evaluate test and if true evaluates then, and if false evaluates else. If else is missing then we just evaluate nil.

(iter (foreach x [3 1 4 1 5 9 2 6 5 3 5 8])
      (if (odd? x)
          (collect x)
          (collect (- x))))

  => (3 1 -4 1 5 9 -2 -6 5 3 5 -8)

This is not the regular Clojure if even though it behaves like one, it is part of the iter DSL. As such, the if variants like if-not are not valid iter expressions.

when

(when test & body)

This is equivalent to (if test (do body)). It's useful for the times when there are multiple collect or accum statements within body,

(iter (foreach x (range 20))
      (when (odd? x)
        (collect (format "%d" x))
        (collect (format "0x%x" x))))

  => ("1" "0x1" "3" "0x3" "5" "0x5" "7" "0x7" "9" "0x9"
      "11" "0xb" "13" "0xd" "15" "0xf" "17" "0x11" "19" "0x13")

do

(do & body)

Just like the normal Clojure do, but modified to support iter. That is, evaluate the body expressions in order.

(iter (foreach x (range 20))
      (if (odd? x)
          (do
            (collect (format "%d" x))
            (collect (format "0x%x" x)))
          (do
            (collect (Integer/toBinaryString x))
            (collect (format "0%o" x)))))

  => ("0" "00" "1" "0x1" "10" "02" "3" "0x3" "100" "04" "5" "0x5"
      "110" "06" "7" "0x7" "1000" "010" "9" "0x9" "1010" "012"
      "11" "0xb" "1100" "014" "13" "0xd" "1110" "016" "15" "0xf"
      "10000" "020" "17" "0x11" "10010" "022" "19" "0x13")

return

(return expr)

Stop execution and return expr.

(iter (foreach x [3 1 4 1 5 9])
      (when (> x 4)
        (return x)))

  => 5

You cannot use both collect and return in the same iter forms.

stop

(stop)

Stop execution of the iter forms.

(iter (foreach x [3 1 4 1 5 9])
      (collect x)
      (when (> x 4)
        (stop)))

  => (3 1 4 1 5)

break

(break)

(break nlevels)

Like the C Language break, stop the iteration of the current loop. If used inside a nested loop, the outer loop keeps executing.

If nlevels is given in a nested loop, break out of that many loops.

(iter (foreach x (range 10))
      (if (> x 4)
          (break))
      (collect x))

  => (0 1 2 3 4)

continue

(continue)

(continue nlevels)

Like the C Language continue, skip this step in the iteration of the current loop.

If nlevels is given and we are in a nested loop, continue with the loop that many levels out.

(iter (foreach x (range 10))
      (if (even? x)
          (continue))
      (collect x))

  => (1 3 5 7 9)

while

(while test)

Run the loop while test is true. This is exactly equivalent to (if (not test) (break))

(iter (foreach x (range 10))
      (while (< x 5))
      (collect x))

  => (0 1 2 3 4)

finding

(finding expr)

(finding expr where)

Return the the first value of expr that is true. If where is given, return the first value of expr that has where true.

(iter (foreach s (clojure.string/split
                   "Lisp is a programmable programming language" #" "))
      (finding (clojure.string/lower-case s) (> (count s) 5)))

  => "programmable"

Binding Operators

let

(let [var init]* & body)

Like the Clojure let expression. Bind var to init for all of body.

(iter (foreach x (range 10))
      (let [y (+ 10 x)]
        (collect y)))

  => (10 11 12 13 14 15 16 17 18 19)

with

(with var init)

The with operator binds var to init for the remainder for the iter form. This is similar to a let expression.

(iter (foreach x (range 10))
      (with y (+ 10 x))
      (with z (+ 100 y))
      (collect z))

  => (110 111 112 113 114 115 116 117 118 119)

with-first

(with-first var)

Sets var to true in the first cycle of the iteration, and false every other time.

(iter (foreach s ["Apple" "Peaches" "Pumpkin Pie"])
      (with-first first?)
      (when (not first?)
        (print ", "))
      (print s))

when evaluated, prints:

"Apple, Peaches, Pumpkin Pie"

with-prev

(with-prev var source-var)

(with-prev var source-var init)

Sets var to the previous value of source-var. In this way you can gain access to the last value of source-var.

If not given, init defaults to nil.

(iter (foreach x (range 5))
      (with-prev y x :start)
      (collect [x y]))))

  => ([0 :start] [1 0] [2 1] [3 2] [4 3])

accum

(accum var expr)

(accum var expr init)

Accumulate var by expr. Binds var for the remainder for the iter form. This is similar to with, but var is persistent through each cycle of the iteration.

accum acts as an alternate to collect to gather values, but where collect creates and returns a lazy sequence accum persists the values across iterations.

If not given, init defaults to nil.

(iter (foreach x (range 10))
      (accum sum (+ sum x) 0)
      (collect sum))

  => (0 1 3 6 10 15 21 28 36 45)

(iter (times 10)
      (with-prev m k 0)
      (accum k (+ m k) 1)
      (collect m))

  => (0 1 1 2 3 5 8 13 21 34)

Aggregate Operators

counting

(counting expr)

Return the count of the number of times expr is true.

(iter (foreach x (range 10))
      (when (odd? x)
        (counting x)))

  => 5

maximizing

(maximizing expr)

(maximizing expr :using key-expr)

Return the maximum value of expr. If key-expr is given maximize by that value.

(iter (foreach [co market-cap] {:aapl 478 :brk 308 :goog 313
                                :msft 340 :xom 422})
      (maximizing co :using market-cap))

  => :aapl

minimizing

(minimizing expr)

(minimizing expr :using key-expr)

Return the minimum value of expr. If key-expr is given minimize by that value.

(iter (foreach [co market-cap] {:aapl 478 :brk 308 :goog 313
                                :msft 340 :xom 422})
      (minimizing co :using market-cap))

  => :brk

summing

(summing expr)

Add up expr each time summing is called and and return the result. summing is initially set to 0 and returns 0 if summing is never executed.

    (iter (foreach x (range 1 101))
          (summing x))

      => 5050


    (iter (foreach x (range 10))
          (when (> x 100)
            (summing x)))

      => 0

multiplying

(multiplying expr)

Multiply expr each time multiplying is called and and return the result. multiplying is initially set to 1 and returns 1 if multiplying is never executed.

(iter (foreach x (range 6))
      (when (odd? x)
        (multiplying x)))

  => 15

(iter (foreach x (range 6))
      (when (> x 100)
        (multiplying x)))

  => 1

reducing

(reducing expr :by by)

(reducing expr :by by :init init)

(reducing expr :by by :finally-by finally-by)

(reducing expr :by by :init init :finally-by finally-by)

The iter version of the Clojure reduce operator. expr is collected and the by function is called using Clojure reduce. If init is given, it is used as the initial value for reduce. If finally-by is given, it is a function of one argument that handles the result of reduce.

(iter (foreach x (range 10))
      (if (even? x)
          (reducing x :init 100 :by +)))

 => 120

Pre and Post Processing Operators

begin

(begin & body)

Evaluate the body expressions before any iteration starts. All accum variables are available in the begin block.

(iter (foreach i (range 5))
                 (begin (println "Start"))
                 (println i))

When evaluated, prints

Start
0
1
2
3
4

end

(end & body)

Evaluate the body expressions after iteration ends. All accum variables are available in the end block.

(iter (foreach i (range 5))
      (end (println "Done"))
      (println i))

when evaluated, prints

0
1
2
3
4
Done

finally-by

(finally-by f)

Apply the single argument function f to the lazy sequence that iter generates.

(iter (foreach x (range 10))
      (when (even? x)
        (collect x))
      (finally-by (fn [xs] (reduce + xs))))

  => 20

iter!

Much in the same way you would use doseq for side effects, iter! can be used for looping side effects.

Although you can wrap the normal iter expression in (dorun (iter ...), you can use iter! for the same purpose. iter! always returns nil.

(iter! (times 3)
       (println "Hello world!"))

  => nil

Also prints:

Hello world!
Hello world!
Hello world!

Writing iter macros

The define-iter-op is used to create new iter operators by defining an iter macro. The only difference between iter macros and regular macros is that iter macros are recursively parsed by iter.

All the normal caveats to writing regular macros apply when writing iter macros, but iter macros have an extra challenge. Because iter can expand multiple macros at once you should always use gensym to create temporary variables and not the # suffix (as in var#).

(define-iter-op my-counting [x]
  (let [counter (gensym "counter-")]
    `(do
       (when ~x
         (accum ~counter (inc ~counter) 0))
       (end (return ~counter)))))

(define-iter-op my-maximizing [x]
  `(iter.core/reducing ~x :by max))

Testing

make test

Authors

Chris Dean ctdean@sokitomi.com

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Iter - An iteration construct for Clojure

Installation

Overview

How it works

Nested Loops

Why?

Operators

Iteration Operators

foreach

forlist

forever

fornext

times

Collection operators

collect

collect-cat

collect-map

collect-uniq

collect-freq

Control Flow Operators

if

when

do

return

stop

break

continue

while

finding

Binding Operators

let

with

with-first

with-prev

accum

Aggregate Operators

counting

maximizing

minimizing

summing

multiplying

reducing

Pre and Post Processing Operators

begin

end

finally-by

iter!

Writing iter macros

Testing

See also

Authors