[hcoop/debian/mlton.git] / doc / guide / src / GenerativeDatatype.adoc

GenerativeDatatype
==================

In <:StandardML:Standard ML>, datatype declarations are said to be
_generative_, because each time a datatype declaration is evaluated,
it yields a new type.  Thus, any attempt to mix the types will lead to
a type error at compile-time.  The following program, which does not
type check, demonstrates this.

[source,sml]
----
functor F () =
   struct
      datatype t = T
   end
structure S1 = F ()
structure S2 = F ()
val _: S1.t -> S2.t = fn x => x
----

Generativity also means that two different datatype declarations
define different types, even if they define identical constructors.
The following program does not type check due to this.

[source,sml]
----
datatype t = A | B
val a1 = A
datatype t = A | B
val a2 = A
val _ = if true then a1 else a2
----

== Also see ==

 * <:GenerativeException:>
Commit	Line	Data
7f918cf1 CE	1	GenerativeDatatype
	2	==================
	3
	4	In <:StandardML:Standard ML>, datatype declarations are said to be
	5	_generative_, because each time a datatype declaration is evaluated,
	6	it yields a new type. Thus, any attempt to mix the types will lead to
	7	a type error at compile-time. The following program, which does not
	8	type check, demonstrates this.
	9
	10	[source,sml]
	11	----
	12	functor F () =
	13	struct
	14	datatype t = T
	15	end
	16	structure S1 = F ()
	17	structure S2 = F ()
	18	val _: S1.t -> S2.t = fn x => x
	19	----
	20
	21	Generativity also means that two different datatype declarations
	22	define different types, even if they define identical constructors.
	23	The following program does not type check due to this.
	24
	25	[source,sml]
	26	----
	27	datatype t = A \| B
	28	val a1 = A
	29	datatype t = A \| B
	30	val a2 = A
	31	val _ = if true then a1 else a2
	32	----
	33
	34	== Also see ==
	35
	36	* <:GenerativeException:>