Sum data type
Data Structure
This illustrates a data structure, a means of storing data within a program.
- Task
Create a sum data type:
A sum data type is a data structure used to hold a value that could take on several different, but fixed, types. Only one of the types can be in use at any one time.
Sum data types are considered an algebraic data type and are also known as tagged union, variant, variant record, choice type, discriminated union, disjoint union or coproduct.
- Related task
- See also
- Array
- Associative array: Creation, Iteration
- Collections
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Set
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Stack
ALGOL 68
Algol 68's UNION MODE allows the definition of items which can have different types. <lang algol68>MODE LEAF = INT; MODE NODE = STRUCT( TREE left, TREE right ); MODE TREE = UNION( VOID, LEAF, REF NODE );
TREE t1 = LOC NODE := ( LEAF( 1 ), LOC NODE := ( LEAF( 2 ), LEAF( 3 ) ) );</lang>
Note that assignment/initialisation of UNION items is just of a matter of specifying the assigned/initial value, as above; however to use the value requires a CASE clause, such as in the example below (which would print "node", given the above declarations).
<lang algol68>CASE t1
IN (REF NODE n): print( ( "node", newline ) ) , ( LEAF l): print( ( "leaf ", l, newline ) ) , ( VOID ): print( ( "empty", newline ) )
ESAC</lang>
C
C has the union data type which can store multiple variables at the same memory location. This was a very handy feature when memory was a scarce commodity. Even now this is an essential feature which enables low level access such as hardware I/O access, word or bitfield sharing thus making C especially suited for Systems Programming.
What follows are two example programs. In both an union stores an integer, a floating point and a character at the same location. If all values are initialized at once, data is corrupted, as shown in the first example. Proper use of unions require storing and retrieving data only when required.
Incorrect usage
<lang C>
- include<stdio.h>
typedef union data{
int i;
float f;
char c;
}united;
int main() {
united udat;
udat.i = 5;
udat.f = 3.14159;
udat.c = 'C';
printf("Integer i = %d , address of i = %p\n",udat.i,&udat.i);
printf("Float f = %f , address of f = %p\n",udat.f,&udat.f);
printf("Character c = %c , address of c = %p\n",udat.c,&udat.c);
return 0;
} </lang> Output :
Integer i = 1078529859 , address of i = 0x7ffc475e3c64 Float f = 3.141557 , address of f = 0x7ffc475e3c64 Character c = C , address of c = 0x7ffc475e3c64
Correct usage
<lang C>
- include<stdio.h>
typedef union data{
int i;
float f;
char c;
}united;
int main() {
united udat;
udat.i = 5;
printf("Integer i = %d , address of i = %p\n",udat.i,&udat.i);
udat.f = 3.14159;
printf("Float f = %f , address of f = %p\n",udat.f,&udat.f);
udat.c = 'C';
printf("Character c = %c , address of c = %p\n",udat.c,&udat.c);
return 0;
} </lang> Output:
Integer i = 5 , address of i = 0x7ffd71122354 Float f = 3.14159 , address of f = 0x7ffd71122354 Character c = C , address of c = 0x7ffd71122354
Delphi
See Pascal
Factor
This is accomplished by defining a tuple with only one slot. The slot should have a class declaration that is a union class. This ensures that the slot may only contain an object of a class that is in the union. A convenient way to do this is with an anonymous union, as in the example below. An explicit UNION: definition may also be used.
In the example below, we define a pseudo-number tuple with one slot that can hold either a number (a built-in class) or a numeric-string — a class which we have defined to be any string that can parse as a number using the string>number word.
<lang factor>USING: accessors kernel math math.parser strings ;
PREDICATE: numeric-string < string string>number >boolean ; TUPLE: pseudo-number { value union{ number numeric-string } } ; C: <pseudo-number> pseudo-number ! constructor
5.245 <pseudo-number> ! ok "-17" >>value ! ok "abc42" >>value ! error</lang>
Free Pascal
See Pascal. The type variant is implemented as a variant record.
Go
Go doesn't natively support sum types, though it's not difficult to create one (albeit verbosely) as the following example shows.
Normally, the IPAddr type (and associated types/methods) would be placed in a separate package so its 'v' field couldn't be accessed directly by code outside that package. However here, for convenience, we place it in the 'main' package. <lang go>package main
import (
"errors" "fmt"
)
type (
IpAddr struct{ v interface{} }
Ipv4 = [4]uint8
Ipv6 = string
)
var zero = Ipv4{}
func NewIpAddr(v interface{}) (*IpAddr, error) {
switch v.(type) {
case Ipv4, Ipv6:
return &IpAddr{v}, nil
default:
err := errors.New("Type of value must either be Ipv4 or Ipv6.")
return nil, err
}
}
func (ip *IpAddr) V4() (Ipv4, error) {
switch ip.v.(type) {
case Ipv4:
return ip.v.(Ipv4), nil
default:
err := errors.New("IpAddr instance doesn't currently hold an Ipv4.")
return zero, err
}
}
func (ip *IpAddr) SetV4(v Ipv4) {
ip.v = v
}
func (ip *IpAddr) V6() (Ipv6, error) {
switch ip.v.(type) {
case Ipv6:
return ip.v.(Ipv6), nil
default:
err := errors.New("IpAddr instance doesn't currently hold an Ipv6.")
return "", err
}
}
func (ip *IpAddr) SetV6(v Ipv6) {
ip.v = v
}
func check(err error) {
if err != nil {
fmt.Println(err)
}
}
func main() {
v4 := Ipv4{127, 0, 0, 1}
ip, _ := NewIpAddr(v4)
home, _ := ip.V4()
fmt.Println(home)
v6 := "::1"
ip.SetV6(v6)
loopback, _ := ip.V6()
fmt.Println(loopback)
_, err := ip.V4()
check(err)
rubbish := 6
ip, err = NewIpAddr(rubbish)
check(err)
}</lang>
- Output:
[127 0 0 1] ::1 IpAddr instance doesn't currently hold an Ipv4. Type of value must either be Ipv4 or Ipv6.
Java
Java does not support sum data types. However, generic data types are supported. An example of generic data types is shown.
<lang Java> import java.util.Arrays;
public class SumDataType {
public static void main(String[] args) {
for ( ObjectStore<?> e : Arrays.asList(new ObjectStore<String>("String"), new ObjectStore<Integer>(23), new ObjectStore<Float>(new Float(3.14159))) ) {
System.out.println("Object : " + e);
}
}
public static class ObjectStore<T> {
private T object;
public ObjectStore(T object) {
this.object = object;
}
@Override
public String toString() {
return "value [" + object.toString() + "], type = " + object.getClass();
}
}
} </lang>
- Output:
Object : value [String], type = class java.lang.String Object : value [23], type = class java.lang.Integer Object : value [3.14159], type = class java.lang.Float
Julia
Julia allows the creation of union types. <lang Julia>
julia> using Sockets # for IP types
julia> MyUnion = Union{Int64, String, Float64, IPv4, IPv6}
Union{Float64, Int64, IPv4, IPv6, String}
julia> arr = MyUnion[2, 4.8, ip"192.168.0.0", ip"::c01e:fc9a", "Hello"]
5-element Array{Union{Float64, Int64, IPv4, IPv6, String},1}:
2
4.8
ip"192.168.0.0"
ip"::c01e:fc9a"
"Hello"
</lang>
OCaml
<lang ocaml>type tree = Empty
| Leaf of int
| Node of tree * tree
let t1 = Node (Leaf 1, Node (Leaf 2, Leaf 3))</lang>
Pascal
<lang pascal>type someOrdinalType = boolean; sumDataType = record case tag: someOrdinalType of false: ( number: integer; ); true: ( character: char; ); end;</lang> Naming a tag can be omitted, but then introspection, i. e. retrieving which alternative is “active”, can not be done. A record can have at most one variant part, which has to appear next to the end of the record definition.
Perl
No native type in Perl for this, use a filter to enforce the rules. <lang perl>use strict; use warnings; use feature 'say';
sub filter {
my($text) = @_;
if (length($text)>1 and $text eq reverse $text) {
return 1, 'Palindromic';
} elsif (0 == length(($text =~ s/\B..*?\b ?//gr) =~ s/^(.)\1+//r)) {
return 1, 'Alliterative';
}
return 0, 'Does not compute';
}
for my $text ('otto', 'ha ha', 'a', 'blue skies', 'tiptoe through the tulips', 12321) {
my($status,$message) = analyze $text; printf "%s $message\n", $status ? 'Yes' : 'No ';
}</lang>
- Output:
Yes Palindromic Yes Alliterative No Does not compute No Does not compute Yes Alliterative Yes Palindromic
Phix
Phix has the object type, which can hold an integer, float, string, (nested) sequence, or anything else you can think of.
User defined types can be used to enforce restrictions on the contents of variables <lang Phix>type ipv4(object o)
if not sequence(o) or length(o)!=4 then
return false
end if
for i=1 to 4 do
if not integer(o[i]) then
return false
end if
end for
return true
end type
type ipv6(object o)
return string(o)
end type
type ipaddr(object o)
return ipv4(o) or ipv6(o)
end type
ipaddr x x = {127,0,0,1} -- fine x = "::c01e:fc9a" -- fine x = -1 -- error</lang>
Raku
(formerly Perl 6) Perl 6 doesn't really have Sum Types as a formal data structure but they can be emulated with enums and switches or multi-dispatch. Note that in this case, feeding the dispatcher an incorrect value results in a hard fault; it doesn't just dispatch to the default. Of course those rules can be relaxed or made more restrictive depending on your particular use case.
<lang perl6>enum Traffic-Signal < Red Yellow Green Blue >;
sub message (Traffic-Signal $light) {
with $light {
when Red { 'Stop!' }
when Yellow { 'Speed Up!' }
when Green { 'Go! Go! Go!' }
when Blue { 'Wait a minute, How did we end up in Japan?!' }
default { 'Whut?' }
}
}
my \Pink = 'A Happy Balloon';
for Red, Green, Blue, Pink -> $signal {
say message $signal;
}</lang>
- Output:
Stop!
Go! Go! Go!
Wait a minute, How did we end up in Japan?!
Type check failed in binding to parameter '$light'; expected Traffic-Signal but got Str ("A Happy Balloon")
REXX
The REXX language is untyped, it is up to the program to decide if it's valid and how to deal with an invalid structure. <lang rexx>/*REXX pgm snipette validates a specific type of data structure, an IP v4 address (list)*/ ip= 127 0 0 1 if val_ipv4(ip) then say 'valid IPV4 type: ' ip
else say '***error*** invalid IPV4 type: ' ip
...
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ val_ipv4: procedure; parse arg $; if words($)\==4 | arg()\==1 then return 0
do j=1 for 4; _=word($, j); #=datatype(_, 'W'); L= length(_)
if verify(_, 0123456789)\==0 | \# | _<0 | _>255 | L>3 then return 0
end /*j*/
return 1 /*returns true (1) if valid, 0 if not. */</lang>
Rust
<lang rust>enum IpAddr {
V4(u8, u8, u8, u8), V6(String),
}
let home = IpAddr::V4(127, 0, 0, 1);
let loopback = IpAddr::V6(String::from("::1"));</lang>
Scala
- Output:
See it yourself by running in your browser either by ScalaFiddle (ES aka JavaScript, non JVM) or Scastie (remote JVM).
<lang Scala>case class Envelop[T](member: T)
val list = List(
Envelop("a string"),
Envelop(732), // an integer
Envelop('☺'), // a character
Envelop(true) // a boolean value
)
list.foreach { case Envelop(element) => println(element) }</lang>
zkl
zkl is untyped - it is up to the container to decide if it wants to deal with a type or not. <lang zkl>ip:=List(127,0,0,1); addrs:=Dictionary("ip",ip);</lang> <lang zkl>class Addr{
fcn init(addr){
var ip = addr;
if(not List.isType(addr)) throw(Exception.TypeError);
}
} ip:=Addr(List(127,0,0,1)); Addr(127,0,0,1); // TypeError : Invalid type Addr(List("abc")); // doesn't fail, would need more error checking ip.ip=L(192,168,1,1); // this doesn't type check either</lang>