Pattern matching

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Pattern matching
You are encouraged to solve this task according to the task description, using any language you may know.

Some languages offer direct support for algebraic data types and pattern matching on them. While this of course can always be simulated with manual tagging and conditionals, it allows for terse code which is easy to read, and can represent the algorithm directly.

As an example, implement insertion in a red-black-tree. A red-black-tree is a binary tree where each internal node has a color attribute red or black. Moreover, no red node can have a red child, and every path from the root to an empty node must contain the same number of black nodes. As a consequence, the tree is balanced, and must be re-balanced after an insertion.


[edit] Bracmat

( ( balance
= a x b y c zd
.  !arg
 : ( B
. ( ( R
. ((R.?a,?x,?b),?y,?c)
| (?a,?x,(R.?b,?y,?c))
, ?zd
| ( ?a
, ?x
, ( R
. ((R.?b,?y,?c),?zd)
| (?b,?y,(R.?c,?zd))
& (R.(B.!a,!x,!b),!y,(B.!c,!zd))
| !arg
& ( ins
= C X tree a m z
.  !arg:(?X.?tree)
& !tree:(?C.?a,?m,?z)
& (  !X:<!m
& balance$(!C.ins$(!X.!a),!m,!z)
|  !X:>!m
& balance$(!C.!a,!m,ins$(!X.!z))
| !tree
| (R.,!X,)
& ( insert
= X tree
.  !arg:(?X.?tree)
& ins$(!X.!tree):(?.?X)
& (B.!X)
& ( insertMany
= L R tree
.  !arg:(%?L_%?R.?tree)
& insertMany$(!L.!tree):?tree
& insertMany$(!R.!tree)
| insert$!arg


(   (   it allows for terse code which is easy to read
, and can represent the algorithm directly
 : ?values
& insertMany$(!values.):?tree
& lst$tree
& done


. ( B
. (R.(B.,,),algorithm,(B.,allows,))
, and
, (B.,can,)
, code
, ( R
. ( B
. (B.(R.,directly,),easy,)
, for
, (B.(R.,is,),it,)
, read
, ( B
. (B.,represent,)
, terse
, (R.(B.,the,),to,(B.,which,))

[edit] Clojure

Pattern matching library: core.match.

For code and a thorough write-up on the red-black tree implementation that uses core.match, please read: Clojure Cookbook - Data Structures: Red-Black Trees.

[edit] Common Lisp

Common Lisp doesn't come with any pattern-matching solutions on its own, but with the help of its macro facility, it can incorporate features from other languages such as pattern matching. Macros expand into efficient code during compilation time and there isn't much difference if it's included in the core language or not. As has been said, Lisp is a ball of mud and remains one no matter what one throws at it.

This is a straighforward translation of the TCL solution. I don't know red-black-trees myself but I tried mirroring the original program as closely as possible. It uses a pattern-matching library called toadstool.

Library: toadstool
(mapc #'use-package '(#:toadstool #:toadstool-system))
(defstruct (red-black-tree (:constructor tree (color left val right)))
color left val right)
(defcomponent tree (operator macro-mixin))
(defexpand tree (color left val right)
`(class red-black-tree red-black-tree-color ,color
red-black-tree-left ,left
red-black-tree-val ,val
red-black-tree-right ,right))
(pushnew 'tree *used-components*)
(defun balance (color left val right)
(toad-ecase (color left val right)
(('black (tree 'red (tree 'red a x b) y c) z d)
(tree 'red (tree 'black a x b) y
(tree 'black c z d)))
(('black (tree 'red a x (tree 'red b y c)) z d)
(tree 'red (tree 'black a x b) y (tree 'black c z d)))
(('black a x (tree 'red (tree 'red b y c) z d))
(tree 'red (tree 'black a x b) y (tree 'black c z d)))
(('black a x (tree 'red b y (tree 'red c z d)))
(tree 'red (tree 'black a x b) y (tree 'black c z d)))
((color a x b)
(tree color a x b))))
(defun %insert (x s)
(toad-ecase1 s
(nil (tree 'red nil x nil))
((tree color a y b)
(cond ((< x y)
(balance color (%insert x a) y b))
((> x y)
(balance color a y (%insert x b)))
(t s)))))
(defun insert (x s)
(toad-ecase1 (%insert x s)
((tree t a y b) (tree 'black a y b))))

[edit] E

Translation of: Haskell

In E, a pattern can be used almost anywhere a variable name can. Additionally, there are two operators used for pattern matching idioms: =~ (returns success as a boolean, somewhat like Perl's =~), and switch (matches multiple patterns, like Haskell's case).

Both of those operators are defined in terms of the basic bind/match operation: def pattern exit failure_handler := specimen

def balance(tree) {
  return if (
    tree =~ term`tree(black, tree(red, tree(red, @a, @x, @b), @y, @c), @z, @d)` ||
    tree =~ term`tree(black, tree(red, @a, @x, tree(red, @b, @y, @c)), @z, @d)` ||
    tree =~ term`tree(black, @a, @x, tree(red, tree(red, @b, @y, @c), @z, @d))` ||
    tree =~ term`tree(black, @a, @x, tree(red, @b, @y, tree(red, @c, @z, @d)))`
  ) {
    term`tree(red, tree(black, $a, $x, $b), $y, tree(black, $c, $z, $d))`
  } else { tree }
def insert(elem, tree) {
  def ins(tree) {
    return switch (tree) {
      match term`empty` { term`tree(red, empty, $elem, empty)` }
      match term`tree(@color, @a, @y, @b)` {
        if (elem < y) {
          balance(term`tree($color, ${ins(a)}, $y, $b)`)
        } else if (elem > y) {
          balance(term`tree($color, $a, $y, ${ins(b)})`)
        } else {
  def term`tree(@_, @a, @y, @b)` := ins(tree)
  return term`tree(black, $a, $y, $b)`

This code was tested by filling a tree with random values; you can try this at the E REPL:

? var tree := term`empty`
> for _ in 1..20 {
>   tree := insert(entropy.nextInt(100), tree)
> }
> tree

[edit] EchoLisp

;; code adapted from Racket and Common Lisp
;; Illustrates matching on structures
(require 'match)
(require 'struct)
(define (N-tostring n) (format "%s %d" (N-color n) (N-value n)))
(struct N (color left value right) #:tostring N-tostring)
(define (balance t)
(match t
[(N '⚫️ (N '🔴 (N '🔴 a x b) y c) z d) (N '🔴 (N '⚫️ a x b) y (N '⚫️ c z d))]
[(N '⚫️ (N '🔴 a x (N '🔴 b y c)) z d) (N '🔴 (N '⚫️ a x b) y (N '⚫️ c z d))]
[(N '⚫️ a x (N '🔴 (N '🔴 b y c) z d)) (N '🔴 (N '⚫️ a x b) y (N '⚫️ c z d))]
[(N '⚫️ a x (N '🔴 b y (N '🔴 c z d))) (N '🔴 (N '⚫️ a x b) y (N '⚫️ c z d))]
[else t]))
(define (ins value: x tree: t)
(match t
['empty (N '🔴 'empty x 'empty)]
[(N c l v r) (cond [(< x v) (balance (N c (ins x l) v r))]
[(> x v) (balance (N c l v (ins x r)))]
[else t])]))
(define (insert value: x tree: s)
(match (ins x s) [(N _ l v r) (N '⚫️ l v r)]))
(define (t-show n (depth 0))
(when (!eq? 'empty n)
(t-show (N-left n) (+ 12 depth))
(writeln (string-pad-left (format "%s" n ) depth))
(t-show (N-right n) (+ 12 depth))))
(define T (for/fold [t 'empty] ([i 32]) (insert (random 100) t)))
(t-show T)

                                                       🔴 1    
                                           ⚫️ 2    
                               ⚫️ 7    
                                           ⚫️ 8    
                                                      🔴 11    
                  🔴 17    
                                          ⚫️ 25    
                              ⚫️ 28    
                                          ⚫️ 31    
      ⚫️ 32    
                              ⚫️ 36    
                  ⚫️ 40    
                              ⚫️ 43    
⚫️ 44    
                                          🔴 45    
                              ⚫️ 53    
                  ⚫️ 71    
                                          🔴 72    
                              ⚫️ 73    
      ⚫️ 83    
                              ⚫️ 89    
                                          🔴 91    
                  ⚫️ 92    
                                          🔴 94    
                              ⚫️ 99    

[edit] Erlang

The code used here is extracted from Mark Northcott's GitHubGist.

-export([insert/3, find/2]).
% Node structure: { Key, Value, Color, Smaller, Bigger }
find(_, nil) ->
find(Key, { Key, Value, _, _, _ }) ->
{ found, { Key, Value } };
find(Key, { Key1, _, _, Left, _ }) when Key < Key1 ->
find(Key, Left);
find(Key, { Key1, _, _, _, Right }) when Key > Key1 ->
find(Key, Right).
insert(Key, Value, Tree) ->
make_black(ins(Key, Value, Tree)).
ins(Key, Value, nil) ->
{ Key, Value, r, nil, nil };
ins(Key, Value, { Key, _, Color, Left, Right }) ->
{ Key, Value, Color, Left, Right };
ins(Key, Value, { Ky, Vy, Cy, Ly, Ry }) when Key < Ky ->
balance({ Ky, Vy, Cy, ins(Key, Value, Ly), Ry });
ins(Key, Value, { Ky, Vy, Cy, Ly, Ry }) when Key > Ky ->
balance({ Ky, Vy, Cy, Ly, ins(Key, Value, Ry) }).
make_black({ Key, Value, _, Left, Right }) ->
{ Key, Value, b, Left, Right }.
balance({ Kx, Vx, b, Lx, { Ky, Vy, r, Ly, { Kz, Vz, r, Lz, Rz } } }) ->
{ Ky, Vy, r, { Kx, Vx, b, Lx, Ly }, { Kz, Vz, b, Lz, Rz } };
balance({ Kx, Vx, b, Lx, { Ky, Vy, r, { Kz, Vz, r, Lz, Rz }, Ry } }) ->
{ Kz, Vz, r, { Kx, Vx, b, Lx, Lz }, { Ky, Vy, b, Rz, Ry } };
balance({ Kx, Vx, b, { Ky, Vy, r, { Kz, Vz, r, Lz, Rz }, Ry }, Rx }) ->
{ Ky, Vy, r, { Kz, Vz, b, Lz, Rz }, { Kx, Vx, b, Ry, Rx } };
balance({ Kx, Vx, b, { Ky, Vy, r, Ly, { Kz, Vz, r, Lz, Rz } }, Rx }) ->
{ Kz, Vz, r, { Ky, Vy, b, Ly, Lz }, { Kx, Vx, b, Rz, Rx } };
balance(T) ->


> rbtree:insert(0,3,nil).
> T1 = rbtree:insert(0,3,nil).
> T2 = rbtree:insert(1,5,T1). 
> T3 = rbtree:insert(2,-1,T2).
> T4 = rbtree:insert(3,7,T3). 
> T5 = rbtree:insert(4,-3,T4).
> T6 = rbtree:insert(5,0,T5). 
> T7 = rbtree:insert(6,-1,T6).
> T8 = rbtree:insert(7,0,T7). 
> rbtree:find(4,T8).

[edit] Haskell

data Color = R | B
data Tree a = E | T Color (Tree a) a (Tree a)
balance :: Color -> Tree a -> a -> Tree a -> Tree a
balance B (T R (T R a x b) y c ) z d = T R (T B a x b) y (T B c z d)
balance B (T R a x (T R b y c)) z d = T R (T B a x b) y (T B c z d)
balance B a x (T R (T R b y c) z d ) = T R (T B a x b) y (T B c z d)
balance B a x (T R b y (T R c z d)) = T R (T B a x b) y (T B c z d)
balance col a x b = T col a x b
insert :: Ord a => a -> Tree a -> Tree a
insert x s = T B a y b where
ins E = T R E x E
ins s@(T col a y b)
| x < y = balance col (ins a) y b
| x > y = balance col a y (ins b)
| otherwise = s
T _ a y b = ins s

[edit] J

J incorporates a symbol data type which, in versions 6.02 and 7.01, J implements directly as a red-black tree. The s: entry in the J dictionary begins Symbols are a data type and are created by the verb s:. Symbols provide a mechanism for searching, sorting, and comparisons more efficient than alternative mechanisms such as boxed strings. Structural, selection, and relational verbs work on symbols. Arithmetic verbs do not work on symbols.

The following code provides dictionary functionality using a red-black tree written in J without symbols.

help=: noun define
red-black tree
Store dictionary in red-black tree. The keys can be any noun.
Left-leaning Red-Black Trees
Robert Sedgewick
Department of Computer Science
Princeton University
insert key;value Inserts item into tree
delete key Deletes item with key from tree
Deletion via the Sedgewick method is fairly simple.
However, I elected to remove the KEY;VALUE pair
rather than change the tree.
find key Returns the associated definition or EMPTY
items any_noun Returns all the items as a rank 1 array of KEY;VALUE pairs
keys any_noun Returns all the keys as a rank 1 array of boxes
values any_noun Returns all the values as a rank 1 array of boxes
J stores all data as arrays.
I chose to use array indexes to implement pointers.
An "index" is a rank 0 length 1 array.
Internal data structure:
T This rank 2 array stores indexes of left and right at each branch point.
C rank 1 array of node color.
H rank 1 array of the hash value of each key.
R rank 0 array stores the root index.
D rank 1 array of boxes. In each box is a rank 2 array of key value
pairs associated with the hash value. Hash collision invokes direct
lookup by key among the keys having same hash.
Additional test idea (done):
Changing the hash to 0: or 2&| rapidly tests
hash collision code for integer keys.
bitand=: (#. 1 0 0 0 1)b.
bitxor=: (#. 1 0 1 1 0)b.
hash=: [: ((4294967295) bitand (bitxor 1201&*))/ 846661 ,~ ,@:(a.&i.)@:":
NB. hash=: ] [ 1&bitand NB. can choose simple hash functions for tests
setup=: 3 : 0
T=: i. 0 2 NB. Tree
H=: D=: C=: i. 0 NB. Hashes, Data, Color
R=: _ NB. Root
'BLACK RED'=: i. 2
flipColors=: monad def 'C=: -.@:{`[`]}&C (, {&T) y'
3 : 0 'test flipColors'
DD=.D=: ,/<@:(;3j1&":)"0 i.3
TT=.T=: _ _,0 2,:_ _
CC=.C=: 1 0 1
RR=.R=: 1
HH=.H=: i.3
flipColors R
assert C -: -. CC
assert HH -: H
assert TT -: T
assert DD -: D
assert RR -: R
getColor=: monad def 'C ({~ :: (BLACK"_))"_ 0 y' NB. y the node
rotateTree=: dyad define NB. x left or right, y node
I=. x <@:(, -.)~ y
X=. I { T NB. x = root.otherside
J=. X <@:, x
T=: (J { T) I} T
T=: y J} T
C=: y (RED ,~ {)`(X , [)`]} C
3 : 0 'test rotateTree'
DD=.D=:,/<@:(;3j1&":)"0 i.5
TT=.T=:_ _,0 2,_ _,1 4,:_ _
CC=.C=:0 1 0 0 0
assert R = rotateTree/0 1 , R
assert DD -: D
assert CC -: C
assert HH -: H
assert TT -: T
insert_privately=: adverb define
ROOT=. m
HASH=. x
ITEM=. y
if. _ -: ROOT do. NB. new key
ROOT=. # H
H=: H , HASH
T=: T , _ _
D=: D , < ,: , ITEM
C=: C , RED
elseif. HASH = ROOT { H do. NB. change a value or hash collision
I=. STACK i.&:({."1) ITEM
STACK=. ITEM <@:(I}`,@.(I = #@])) STACK
elseif. do. NB. Follow tree
NB. if both children are red then flipColors ROOT
flipColors^:((,~ RED) -: getColor@:({&T)) ROOT
I=. <@:(, HASH > {&H) ROOT
TEMP=. HASH (I { T) insert_privately y
T=: TEMP I } T
NB.if (isRed(h.right) && !isRed(h.left)) h = rotateLeft(h)
ROOT=. 0&rotateTree^:((BLACK,RED) -: getColor@:({&T)) ROOT
NB.if (isRed(h.left) && isRed(h.left.left)) h = rotateRight(h)
if. RED -: getColor {. ROOT { T do.
if. (RED -: (getColor@:(([: {&T <@:,&0)^:2) :: (BLACK"_))) ROOT do.
ROOT=. 1 rotateTree ROOT
insert=: monad define"1
assert 'boxed' -: datatype y
R=: (R insert_privately~ hash@:(0&{::)) y
find_hash_index=: monad define NB. y is the hash
if. 0 = # T do. '' return. end. NB. follow the tree
I=. R NB. instead of
while. y ~: I { H do. NB. direct search
J=. <@:(, y > {&H) I
if. _ > II=. J { T do. I=. II else. '' return. end.
find=: monad define
if. '' -: I=. find_hash_index hash y do. EMPTY return. end.
LIST=. I {:: D
K=. {. |: LIST
LIST {::~ ::empty 1 ,~ K i. < y
delete=: 3 : 0
if. '' -: I=. find_hash_index hash y do. EMPTY return. end.
LIST=. I {:: D
K=. {. |: LIST
J=. K i. < y
RESULT=. J ({::~ ,&1)~ LIST
STACK=. J <@:({. , (}.~ >:)~) LIST
D=. LIST I } D
getPathsToLeaves=: a:&$: : (4 : 0) NB. PATH getPathsToLeaves ROOT use: getPathsToLeaves R
if. 0 = # y do. getPathsToLeaves R return. end.
PATH=. x ,&.> y
if. _ -: y do. return. end.
PATH getPathsToLeaves"0 y { T
check=: 3 : 0
COLORS=. getColor"0&.> a: -.~ ~. , getPathsToLeaves ''
result=. EMPTY
if. 0&e.@:(= {.) +/@:(BLACK&=)@>COLORS do. result=. result,<'mismatched black count' end.
if. 1 e. 1&e.@:(*. (= 1&|.))@:(RED&=)@>COLORS do. result=. result,<'successive reds' end.
getPath=: 3 : 0 NB. get path to y, the key
if. 0 = # H do. EMPTY return. end.
HASH=. hash y
PATH=. , I=. R
while. HASH ~: I { H do.
J=. <@:(, HASH > {&H) I
PATH=. PATH , II=. J { T
if. _ > II do. I=. II else. EMPTY return. end.
items=: 3 :';D'
keys=: 3 :'0{"1 items y'
values=: 3 :'1{"1 items y'

With use:

NB. populate dictionary in random order with 999 key value pairs
insert@:(; 6j1&":)"0@:?~ 999
find 'the' NB. 'the' has no entry.
find 239 NB. entry 239 has the anticipated formatted string value.
find 823823 NB. also no such entry
NB. tree passes the "no consecutive red" and "same number of black"
NB. nodes to and including NULL leaves.

[edit] OCaml

type color = R | B
type 'a tree = E | T of color * 'a tree * 'a * 'a tree
(** val balance : color * 'a tree * 'a * 'a tree -> 'a tree *)
let balance = function
| B, T (R, T (R,a,x,b), y, c), z, d
| B, T (R, a, x, T (R,b,y,c)), z, d
| B, a, x, T (R, T (R,b,y,c), z, d)
| B, a, x, T (R, b, y, T (R,c,z,d)) -> T (R, T (B,a,x,b), y, T (B,c,z,d))
| col, a, x, b -> T (col, a, x, b)
(** val insert : 'a -> 'a tree -> 'a tree *)
let insert x s =
let rec ins = function
| E -> T (R,E,x,E)
| T (col,a,y,b) as s ->
if x < y then
balance (col, ins a, y, b)
else if x > y then
balance (col, a, y, ins b)
in let T (_,a,y,b) = ins s
in T (B,a,y,b)

[edit] Oz

Translation of: Haskell

Unlike Haskell, Oz does not support multiple equations per function. So we use an explicit case-statement. To match multiple variables at once, we create temporary tuples with "#".

fun {Balance Col A X B}
case Col#A#X#B
of b#t(r t(r A X B) Y C )#Z#D then t(r t(b A X B) Y t(b C Z D))
[] b#t(r A X t(r B Y C))#Z#D then t(r t(b A X B) Y t(b C Z D))
[] b#A #X#t(r t(r B Y C) Z D) then t(r t(b A X B) Y t(b C Z D))
[] b#A #X#t(r B Y t(r C Z D)) then t(r t(b A X B) Y t(b C Z D))
else t(Col A X B)
fun {Insert X S}
fun {Ins S}
case S of e then t(r e X e)
[] t(Col A Y B) then
if X < Y then {Balance Col {Ins A} Y B}
elseif X > Y then {Balance Col A Y {Ins B}}
else S
t(_ A Y B) = {Ins S}
t(b A Y B)

[edit] Perl

Works with: Perl version 5.010

Although Perl does not have algebraic data types, it does have a wonderfully flexible regular expression engine, which is powerfully enough to perform the task.

However, representing a tree as a string, and repeatedly parsing that string, is truly inefficient way to solve the problem. Someday, someone will write a perl multi-method-dispatch module which is as amazing as perl6's, and then we can copy the perl6 solution here.

The $balanced variable matches against either some data, or the empty tree (_), or, using perl's amazing recursive regular expression feature, a non-empty tree.

Each of the single letter variables declared right after $balanced, match an instance of $balanced, and if they succeed, store the result into the %+ hash.

use 5.010;
use strict;
use warnings qw(FATAL all);
my $balanced = qr{([^<>,]++|<(?-1),(?-1),(?-1),(?-1)>)};
my ($a, $b, $c, $d, $x, $y, $z) = map +qr((?<$_>$balanced)),
'a'..'d', 'x'..'z';
my $col = qr{(?<col>[RB])};
sub balance {
local $_ = shift;
if( /^<B,<R,<R,$a,$x,$b>,$y,$c>,$z,$d>\z/ or
/^<B,<R,$a,$x,<R,$b,$y,$c>>,$z,$d>\z/ or
/^<B,$a,$x,<R,<R,$b,$y,$c>,$z,$d>>\z/ or
/^<B,$a,$x,<R,$b,$y,<R,$c,$z,$d>>>\z/ )
my ($aa, $bb, $cc, $dd) = @+{'a'..'d'};
my ($xx, $yy, $zz) = @+{'x'..'z'};
} else {
sub ins {
my ($xx, $tree) = @_;
if($tree =~ m{^<$col,$a,$y,$b>\z} ) {
my ($color, $aa, $bb, $yy) = @+{qw(col a b y)};
if( $xx < $yy ) {
return balance "<$color,".ins($xx,$aa).",$yy,$bb>";
} elsif( $xx > $yy ) {
return balance "<$color,$aa,$yy,".ins($xx,$bb).">";
} else {
return $tree;
} elsif( $tree !~ /,/) {
return "<R,_,$xx,_>";
} else {
print "Unexpected failure!\n";
print "Tree parts are: \n";
print $_, "\n" for $tree =~ /$balanced/g;
sub insert {
my $tree = ins(@_);
$tree =~ m{^<$col,$a,$y,$b>\z} or die;
my @a = 1..10;
for my $aa ( 1 .. $#a ) {
my $bb = int rand( 1 + $aa );
@a[$aa, $bb] = @a[$bb, $aa];
my $t = "!";
for( @a ) {
$t = insert( $_, $t );
print "Tree: $t.\n";
print "Done\n";
Tree: <B,_,9,_>.
Tree: <B,<R,_,7,_>,9,_>.
Tree: <B,<B,_,2,_>,7,<B,_,9,_>>.
Tree: <B,<B,_,2,<R,_,6,_>>,7,<B,_,9,_>>.
Tree: <B,<B,_,2,<R,_,6,_>>,7,<B,_,9,<R,_,10,_>>>.
Tree: <B,<R,<B,_,2,_>,5,<B,_,6,_>>,7,<B,_,9,<R,_,10,_>>>.
Tree: <B,<R,<B,_,2,<R,_,4,_>>,5,<B,_,6,_>>,7,<B,_,9,<R,_,10,_>>>.
Tree: <B,<R,<B,_,2,<R,_,4,_>>,5,<B,_,6,_>>,7,<B,<R,_,8,_>,9,<R,_,10,_>>>.
Tree: <B,<R,<B,<R,_,1,_>,2,<R,_,4,_>>,5,<B,_,6,_>>,7,<B,<R,_,8,_>,9,<R,_,10,_>>>.
Tree: <B,<B,<B,<R,_,1,_>,2,_>,3,<B,_,4,_>>,5,<B,<B,_,6,_>,7,<B,<R,_,8,_>,9,<R,_,10,_>>>>.

[edit] Perl 6

Works with: Rakudo version 2012.05

Perl 6 doesn't have algebraic data types (yet), but it does have pretty good pattern matching in multi signatures.

enum RedBlack <R B>;
multi balance(B,[R,[R,$a,$x,$b],$y,$c],$z,$d) { [R,[B,$a,$x,$b],$y,[B,$c,$z,$d]] }
multi balance(B,[R,$a,$x,[R,$b,$y,$c]],$z,$d) { [R,[B,$a,$x,$b],$y,[B,$c,$z,$d]] }
multi balance(B,$a,$x,[R,[R,$b,$y,$c],$z,$d]) { [R,[B,$a,$x,$b],$y,[B,$c,$z,$d]] }
multi balance(B,$a,$x,[R,$b,$y,[R,$c,$z,$d]]) { [R,[B,$a,$x,$b],$y,[B,$c,$z,$d]] }
multi balance($col, $a, $x, $b) { [$col, $a, $x, $b] }
multi ins( $x, @s [$col, $a, $y, $b] ) {
when $x before $y { balance $col, ins($x, $a), $y, $b }
when $x after $y { balance $col, $a, $y, ins($x, $b) }
default { @s }
multi ins( $x, Any:U ) { [R, Any, $x, Any] }
multi insert( $x, $s ) {
[B, ins($x,$s)[1..3]];
sub MAIN {
my $t = Any;
$t = insert($_, $t) for (1..10).pick(*);
say $t.perl;

This code uses generic comparison operators before and after, so it should work on any ordered type.

[RedBlack::B, [RedBlack::R, [RedBlack::B, [RedBlack::R, Any, 1, Any], 2, Any], 3, [RedBlack::B, Any, 4, [RedBlack::R, Any, 5, Any]]], 6, [RedBlack::R, [RedBlack::B, Any, 7, Any], 8, [RedBlack::B, Any, 9, [RedBlack::R, Any, 10, Any]]]]

[edit] PicoLisp

Translation of: Prolog
(be color (R))
(be color (B))
(be tree (@ E))
(be tree (@P (T @C @L @X @R))
(color @C)
(tree @P @L)
(call @P @X)
(tree @P @R) )
(be bal (B (T R (T R @A @X @B) @Y @C) @Z @D (T R (T B @A @X @B) @Y (T B @C @Z @D))))
(be bal (B (T R @A @X (T R @B @Y @C)) @Z @D (T R (T B @A @X @B) @Y (T B @C @Z @D))))
(be bal (B @A @X (T R (T R @B @Y @C) @Z @D) (T R (T B @A @X @B) @Y (T B @C @Z @D))))
(be bal (B @A @X (T R @B @Y (T R @C @Z @D)) (T R (T B @A @X @B) @Y (T B @C @Z @D))))
(be balance (@C @A @X @B @S)
(bal @C @A @X @B @S)
T )
(be balance (@C @A @X @B (T @C @A @X @B)))
(be ins (@X E (T R E @X E)))
(be ins (@X (T @C @A @Y @B) @R)
(^ @ (> (-> @Y) (-> @X)))
(ins @X @A @Ao)
(balance @C @Ao @Y @B @R)
T )
(be ins (@X (T @C @A @Y @B) @R)
(^ @ (> (-> @X) (-> @Y)))
(ins @X @B @Bo)
(balance @C @A @Y @Bo @R)
T )
(be ins (@X (T @C @A @Y @B) (T @C @A @Y @B)))
(be insert (@X @S (T B @A @Y @B))
(ins @X @S (T @ @A @Y @B)) )


: (? (insert 2 E @A) (insert 1 @A @B) (insert 3 @B @C))
@A=(T B E 2 E) @B=(T B (T R E 1 E) 2 E) @C=(T B (T R E 1 E) 2 (T R E 3 E))
-> NIL

[edit] Prolog


tree(P,t(C,L,X,R)) :- color(C), tree(P,L), call(P,X), tree(P,R).

bal(b, t(r,t(r,A,X,B),Y,C), Z, D, t(r,t(b,A,X,B),Y,t(b,C,Z,D))).
bal(b, t(r,A,X,t(r,B,Y,C)), Z, D, t(r,t(b,A,X,B),Y,t(b,C,Z,D))).
bal(b, A, X, t(r,t(r,B,Y,C),Z,D), t(r,t(b,A,X,B),Y,t(b,C,Z,D))).
bal(b, A, X, t(r,B,Y,t(r,C,Z,D)), t(r,t(b,A,X,B),Y,t(b,C,Z,D))).

balance(C,A,X,B,S) :- ( bal(C,A,X,B,T) -> S = T ; S = t(C,A,X,B) ).

ins(X,t(C,A,Y,B),R) :- ( X < Y -> ins(X,A,Ao), balance(C,Ao,Y,B,R)
                       ; X > Y -> ins(X,B,Bo), balance(C,A,Y,Bo,R)
                       ; X = Y -> R = t(C,A,Y,B)

insert(X,S,t(b,A,Y,B)) :- ins(X,S,t(_,A,Y,B)).

[edit] Racket

Translation of: OCaml
#lang racket
;; Using short names to make the code line up nicely
(struct N (color left value right) #:prefab)
(define (balance t)
(match t
[(N 'B (N 'R (N 'R a x b) y c) z d) (N 'R (N 'B a x b) y (N 'B c z d))]
[(N 'B (N 'R a x (N 'R b y c)) z d) (N 'R (N 'B a x b) y (N 'B c z d))]
[(N 'B a x (N 'R (N 'R b y c) z d)) (N 'R (N 'B a x b) y (N 'B c z d))]
[(N 'B a x (N 'R b y (N 'R c z d))) (N 'R (N 'B a x b) y (N 'B c z d))]
[else t]))
(define (insert x s)
(define (ins t)
(match t
['empty (N 'R 'empty x 'empty)]
[(N c l v r) (cond [(< x v) (balance (N c (ins l) v r))]
[(> x v) (balance (N c l v (ins r)))]
[else t])]))
(match (ins s) [(N _ l v r) (N 'B l v r)]))
(define (visualize t0)
(let loop ([t t0] [last? #t] [indent '()])
(define (I mid last) (cond [(eq? t t0) ""] [last? mid] [else last]))
(for-each display (reverse indent))
(printf "~a~a[~a]\n" (I "\\-" "+-") (N-value t) (N-color t))
(define subs (filter N? (list (N-left t) (N-right t))))
(for ([s subs] [n (in-range (sub1 (length subs)) -1 -1)])
(loop s (zero? n) (cons (I " " "| ") indent)))))
(visualize (for/fold ([t 'empty]) ([i 16]) (insert i t)))
| +-1[B]
| | +-0[B]
| | \-2[B]
| \-5[B]
|   +-4[B]
|   \-6[B]
  | +-8[B]
  | \-10[B]

[edit] Rascal

Rascal offers many options for pattern matching. In essence, there are four sorts of patterns: Abstract, Concrete, PatternWithAction and classic Regular Expressions. These patterns can be used in several cases, for example switch or visit statements, on the right of the Match operator (:=), or in TryCatch statements for thrown exceptions. Each pattern binds variables in a conditional scope.

[edit] Abstract

An abstract pattern is recursively defined and may contain, among others, the following elements: Literal, VariableDeclaration, MultiVariable, Variable, List, Set, Tuple, Node, Descendant, Labelled, TypedLabelled, TypeConstrained. More explanation can be found in the Documentation. Some examples:

// Literal
rascal>123 := 123
bool: true
// VariableDeclaration
rascal>if(str S := "abc")
>>>>>>> println("Match succeeds, S == \"<S>\"");
Match succeeds, S == "abc"
// MultiVariable
rascal>if([10, N*, 50] := [10, 20, 30, 40, 50])
>>>>>>> println("Match succeeds, N == <N>");
Match succeeds, N == [20,30,40]
// Variable
rascal>N = 10;
int: 10
rascal>N := 10;
bool: true
rascal>N := 20;
bool: false
// Set and List
rascal>if({10, set[int] S, 50} := {50, 40, 30, 20, 10})
>>>>>>> println("Match succeeded, S = <S>");
Match succeeded, S = {30,40,20}
rascal>for([L1*, L2*] := [10, 20, 30, 40, 50])
>>>>>>> println("<L1> and <L2>");
[] and [10,20,30,40,50]
[10] and [20,30,40,50]
[10,20] and [30,40,50]
[10,20,30] and [40,50]
[10,20,30,40] and [50]
[10,20,30,40,50] and []
list[void]: []
// Descendant
rascal>T = red(red(black(leaf(1), leaf(2)), black(leaf(3), leaf(4))), black(leaf(5), leaf(4)));
rascal>for(/black(_,leaf(4)) := T)
>>>>>>> println("Match!");
list[void]: []
rascal>for(/black(_,leaf(int N)) := T)
>>>>>>> println("Match <N>");
Match 2
Match 4
Match 4
list[void]: []
rascal>for(/int N := T)
>>>>>>> append N;
list[int]: [1,2,3,4,5,4]
// Labelled
rascal>for(/M:black(_,leaf(4)) := T)
>>>>>>> println("Match <M>");
Match black(leaf(3),leaf(4))
Match black(leaf(5),leaf(4))
list[void]: []

[edit] Concrete

Suppose we want to manipulate text written in some hypothetical language LANG. Then first the concrete syntax of LANG has to be defined by importing a module that declares the non-terminals and syntax rules for LANG. Next LANG programs have to be parsed. LANG programs made come from text files or they may be included in the Rascal program as literals. In both cases the text is parsed according to the defined syntax and the result is a parse tree in the form of a value of type Tree. Concrete patterns operate on these trees.

A concrete pattern is a quoted concrete syntax fragment that may contain variables. The syntax that is used to parse the concrete pattern may come from any module that has been imported in the module in which the concrete pattern occurs. Some examples of concrete patterns:

// Quoted pattern 
` Token1 Token2 ... Tokenn `
// A typed quoted pattern
(Symbol) ` Token1 Token2 ... TokenN `
// A typed variable pattern
<Type Var>
// A variable pattern

A full example of concrete patterns can be found in the Rascal Recipes.

[edit] PatternWithAction

There are two variants of the PatternsWitchAction. When the subject matches Pattern, the expression Exp is evaluated and the subject is replaced with the result. Secondly, when the subject matches Pattern, the (block of) Statement(s) is executed. See below for some ColoredTree examples:

// Define ColoredTrees with red and black nodes and integer leaves
data ColoredTree = leaf(int N)
| red(ColoredTree left, ColoredTree right)
| black(ColoredTree left, ColoredTree right);
// Count the number of black nodes
public int cntBlack(ColoredTree t){
int c = 0;
visit(t) {
case black(_,_): c += 1;
return c;
// Returns if a tree is balanced
public bool balance(ColoredTree t){
case black(a,b): if (cntBlack(a) == cntBlack(b)) true; else return false;
case red(a,b): if (cntBlack(a) == cntBlack(b)) true; else return false;
return true;
// Compute the sum of all integer leaves
public int addLeaves(ColoredTree t){
int c = 0;
visit(t) {
case leaf(int N): c += N;
return c;
// Add green nodes to ColoredTree
data ColoredTree = green(ColoredTree left, ColoredTree right);
// Transform red nodes into green nodes
public ColoredTree makeGreen(ColoredTree t){
return visit(t) {
case red(l, r) => green(l, r)

[edit] Regular Expressions

Regular expressions are noated between two slashes. Most normal regular expressions patterns are available, such as ., \n, \d, etc. Additionally, flags can be used to create case intensiveness.

rascal>/XX/i := "some xx";
bool: true
rascal>/a.c/ := "abc";
bool: true

[edit] Scala

Translation of: Haskell

Algebraic data types are implemented in Scala through the combination of a number of different features, to ensure principles of Object Oriented Programming.

The main type is usually defined as a sealed abstract class, which ensures it can't be instantiated, and guarantees that it can't be expanded outside the file it was defined at. This last feature is used so the compiler can verify that the pattern matching is complete, or warn when there are missing cases. It can be ommitted if preferred.

Each subtype is defined either as a case object, for non-paremeterized types, or case class, for parameterized types, all extending the main type. The case keyword is not required, but, when used, it provides a number of default methods which ensure they can be used without any further definitions.

The most important of those default methods for the purpose of algebraic data types is the extractor, a method called either unapply or unapplySeq, and which returns an Option containing the deconstructed parameters, or None if the passed object can't be deconstructed by this method. Scala uses the extractors to implement pattern matching without exposing the internal representation of the data.

This specific task is made much harder than necessary because Scala doesn't have a variant ordering class. Given that limitation, one has to either give up on a singleton object representing the empty tree, or give up on parameterizing the tree itself.

The solution below, uses the latter approach. The algebraic data types are members of a RedBlackTree class, which, itself, receives a type parameter for the keys of the tree, and an implicit parameter for an Ordering for that type. To use the tree it is thus necessary to instantiate an object of type RedBlackTree, and then reference the members of that object.

class RedBlackTree[A](implicit ord: Ordering[A]) {
sealed abstract class Color
case object R extends Color
case object B extends Color
sealed abstract class Tree {
def insert(x: A): Tree = ins(x) match {
case T(_, a, y, b) => T(B, a, y, b)
case E => E
def ins(x: A): Tree
case object E extends Tree {
override def ins(x: A): Tree = T(R, E, x, E)
case class T(c: Color, left: Tree, a: A, right: Tree) extends Tree {
private def balance: Tree = (c, left, a, right) match {
case (B, T(R, T(R, a, x, b), y, c), z, d ) => T(R, T(B, a, x, b), y, T(B, c, z, d))
case (B, T(R, a, x, T(R, b, y, c)), z, d ) => T(R, T(B, a, x, b), y, T(B, c, z, d))
case (B, a, x, T(R, T(R, b, y, c), z, d )) => T(R, T(B, a, x, b), y, T(B, c, z, d))
case (B, a, x, T(R, b, y, T(R, c, z, d))) => T(R, T(B, a, x, b), y, T(B, c, z, d))
case _ => this
override def ins(x: A): Tree =, a) match {
case -1 => T(c, left ins x, a, right ).balance
case 1 => T(c, left, a, right ins x).balance
case 0 => this

Usage example:

scala> val rbt = new RedBlackTree[Int]
rbt: RedBlackTree[Int] = RedBlackTree@17dfcf1

scala> import rbt._
import rbt._

scala> List.range(1, 17).foldLeft(E: Tree)(_ insert _)
res5: rbt.Tree = T(B,T(B,T(B,T(B,E,1,E),2,T(B,E,3,E)),4,T(B,T(B,E,5,E),6,T(B,E,7,E))),8,T(B,T(B,T(B,E,9,E),10,T(B,E,11,E

[edit] Standard ML

datatype color = R | B
datatype 'a tree = E | T of color * 'a tree * 'a * 'a tree
(** val balance = fn : color * 'a tree * 'a * 'a tree -> 'a tree *)
fun balance (B, T (R, T (R,a,x,b), y, c), z, d) = T (R, T (B,a,x,b), y, T (B,c,z,d))
| balance (B, T (R, a, x, T (R,b,y,c)), z, d) = T (R, T (B,a,x,b), y, T (B,c,z,d))
| balance (B, a, x, T (R, T (R,b,y,c), z, d)) = T (R, T (B,a,x,b), y, T (B,c,z,d))
| balance (B, a, x, T (R, b, y, T (R,c,z,d))) = T (R, T (B,a,x,b), y, T (B,c,z,d))
| balance (col, a, x, b) = T (col, a, x, b)
(** val insert = fn : int -> int tree -> int tree *)
fun insert x s = let
fun ins E = T (R,E,x,E)
| ins (s as T (col,a,y,b)) =
if x < y then
balance (col, ins a, y, b)
else if x > y then
balance (col, a, y, ins b)
val T (_,a,y,b) = ins s
T (B,a,y,b)

[edit] Swift

Works with: Swift version 2+
enum Color { case R, B }
enum Tree<A> {
case E
indirect case T(Color, Tree<A>, A, Tree<A>)
func balance<A>(input: (Color, Tree<A>, A, Tree<A>)) -> Tree<A> {
switch input {
case let (.B, .T(.R, .T(.R,a,x,b), y, c), z, d): return .T(.R, .T(.B,a,x,b), y, .T(.B,c,z,d))
case let (.B, .T(.R, a, x, .T(.R,b,y,c)), z, d): return .T(.R, .T(.B,a,x,b), y, .T(.B,c,z,d))
case let (.B, a, x, .T(.R, .T(.R,b,y,c), z, d)): return .T(.R, .T(.B,a,x,b), y, .T(.B,c,z,d))
case let (.B, a, x, .T(.R, b, y, .T(.R,c,z,d))): return .T(.R, .T(.B,a,x,b), y, .T(.B,c,z,d))
case let (col, a, x, b)  : return .T(col, a, x, b)
func insert<A : Comparable>(x: A, s: Tree<A>) -> Tree<A> {
func ins(s: Tree<A>) -> Tree<A> {
switch s {
case .E  : return .T(.R,.E,x,.E)
case let .T(col,a,y,b):
if x < y {
return balance((col, ins(a), y, b))
} else if x > y {
return balance((col, a, y, ins(b)))
} else {
return s
switch ins(s) {
case let .T(_,a,y,b): return .T(.B,a,y,b)
case .E:
return .E

[edit] Tcl

Translation of: Haskell

Tcl doesn't have algebraic types built-in, but they can be simulated using tagged lists, and a custom pattern matching control structure can be built:

# From
package require Tcl 8.5
package provide datatype 0.1
namespace eval ::datatype {
namespace export define match matches
namespace ensemble create
# Datatype definitions
proc define {type = args} {
set ns [uplevel 1 { namespace current }]
foreach cons [split [join $args] |] {
set name [lindex $cons 0]
set args [lrange $cons 1 end]
proc $ns\::$name $args [format {
lreplace [info level 0] 0 0 %s
} [list $name]]
return $type
# Pattern matching
# matches pattern value envVar --
# Returns 1 if value matches pattern, else 0
# Binds match variables in envVar
proc matches {pattern value envVar} {
upvar 1 $envVar env
if {[var? $pattern]} { return [bind env $pattern $value] }
if {[llength $pattern] != [llength $value]} { return 0 }
if {[lindex $pattern 0] ne [lindex $value 0]} { return 0 }
foreach pat [lrange $pattern 1 end] val [lrange $value 1 end] {
if {![matches $pat $val env]} { return 0 }
return 1
# A variable starts with lower-case letter or _. _ is a wildcard.
proc var? term { string match {[a-z_]*} $term }
proc bind {envVar var value} {
upvar 1 $envVar env
if {![info exists env]} { set env [dict create] }
if {$var eq "_"} { return 1 }
dict set env $var $value
return 1
proc match args {
#puts "MATCH: $args"
set values [lrange $args 0 end-1]
set choices [lindex $args end]
append choices \n [list return -code error -level 2 "no match for $values"]
set f [list values $choices [namespace current]]
lassign [apply $f $values] env body
#puts "RESULT: $env -> $body"
dict for {k v} $env { upvar 1 $k var; set var $v }
catch { uplevel 1 $body } msg opts
dict incr opts -level
return -options $opts $msg
proc case args {
upvar 1 values values
set patterns [lrange $args 0 end-2]
set body [lindex $args end]
set env [dict create]
if {[llength $patterns] != [llength $values]} { return }
foreach pattern $patterns value $values {
if {![matches $pattern $value env]} { return }
return -code return [list $env $body]
proc default body { return -code return [list {} $body] }

We can then code our solution similar to Haskell:

datatype define Color = R | B
datatype define Tree = E | T color left val right
# balance :: Color -> Tree a -> a -> Tree a -> Tree a
proc balance {color left val right} {
datatype match $color $left $val $right {
case B [T R [T R a x b] y c] z d -> { T R [T B $a $x $b] $y [T B $c $z $d] }
case B [T R a x [T R b y c]] z d -> { T R [T B $a $x $b] $y [T B $c $z $d] }
case B a x [T R [T R b y c] z d] -> { T R [T B $a $x $b] $y [T B $c $z $d] }
case B a x [T R b y [T R c z d]] -> { T R [T B $a $x $b] $y [T B $c $z $d] }
case col a x b -> { T $col $a $x $b }
# insert :: Ord a => a -> Tree a -> Tree a
proc insert {x s} {
datatype match [ins $x $s] {
case [T _ a y b] -> { T B $a $y $b }
# ins :: Ord a => a -> Tree a -> Tree a
proc ins {x s} {
datatype match $s {
case E -> { T R E $x E }
case [T col a y b] -> {
if {$x < $y} { return [balance $col [ins $x $a] $y $b] }
if {$x > $y} { return [balance $col $a $y [ins $x $b]] }
return $s
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