Algebraic Data Types
Thus far, we have seen variants simply as enumerating a set of constant values, such as:
type day = Sun | Mon | Tue | Wed | Thu | Fri | Sat type ptype = TNormal | TFire | TWater type peff = ENormal | ENotVery | Esuper
But variants are far more powerful that this.
As a running example, here is a variant type that does more than just enumerate values:
type shape = | Point of point | Circle of point * float (* center and radius *) | Rect of point * point (* lower-left and upper-right corners *)
shape, represents a shape that is either a point, a circle,
or a rectangle. A point is represented by a constructor
carries some additional data, which is a value of type
A circle is represented by a constructor
Circle that carries
a pair of type
point * float, which according to the comment
represents the center of the circle and its radius. A rectangle
is represented by a constructor
Rect that carries a pair of type
Here are a couple functions that use the
let area = function | Point _ -> 0.0 | Circle (_,r) -> pi *. (r ** 2.0) | Rect ((x1,y1),(x2,y2)) -> let w = x2 -. x1 in let h = y2 -. y1 in w *. h let center = function | Point p -> p | Circle (p,_) -> p | Rect ((x1,y1),(x2,y2)) -> ((x2 +. x1) /. 2.0, (y2 +. y1) /. 2.0)
shape variant type is the same as those we've seen before in that
it is defined in terms of a collection of constructors. What's different
than before is that those constructors carry additional data along with them.
Every value of type
shape is formed from exactly one of those constructors.
Sometimes we call the constructor a tag, because it tags the data it carries
as being from that particular constructor.
Variant types are sometimes called tagged unions. Every value of the type
is from the set of values that is the union of all values from the underlying
types that the constructor carries. For the
shape type, every value
is tagged with either
Rect and carries a value
from the set of all
point valued unioned with the set of all
values unioned with the set of all
Another name for these variant types is an algebraic data type. "Algebra" here refers to the fact that variant types contain both sum and product types, as defined in the previous lecture. The sum types come from the fact that a value of a variant is formed by one of the constructors. The product types come from that fact that a constructor can carry tuples or records, whose values have a sub-value from each of their component types.
Using variants, we can express a type that represents the union of several
other types, but in a type-safe way. Here, for example, is a type that
represents either a
string or an
type string_or_int = | String of string | Int of int
If we wanted to, we could use this type to code up lists (e.g.) that contain either strings or ints:
type string_or_int_list = string_or_int list let rec sum : string_or_int list -> int = function |  -> 0 | (String s)::t -> int_of_string s + sum t | (Int i)::t -> i + sum t let three = sum [String "1"; Int 2]
Variants thus provide a type-safe way of doing something that might before have seemed impossible.
Variants also make it possible to discriminate which tag a value was constructed with, even if multiple constructors carry the same type. For example:
type t = Left of int | Right of int let x = Left 1 let double_right = function | Left i -> i | Right i -> 2*i
To define a variant type:
type t = C1 [of t1] | ... | Cn [of tn]
The square brackets above denote the type
of ti is optional. Every
constructor may individually either carry no data or carry data.
We call constructors that carry no data constant; and those that
carry data, non-constant.
To write an expression that is a variant:
C e ---or--- C
depending on whether the constructor name
C is non-constant or constant.
C e ==> C v, assuming
Cis already a value, assuming
t = ... | C | ...then
C : t.
t = ... | C of t' | ...and if
e : t'then
C e : t.
We add the following new pattern form to the list of legal patterns:
And we extend the definition of when a pattern matches a value and produces a binding as follows:
vand produces bindings \(b\), then
C vand produces bindings \(b\).