choice.h

Choice types: (tag, ...).

A choice type, also known as tagged union, is represented as (tag, ...), where tag is the type of a value and ... is the value. Perhaps the most common example of a choice type is a binary tree:

[examples/binary_tree.c]

// Sums all nodes of a binary tree, recursively.

#include <metalang99.h>

#define leaf(x)              ML99_choice(v(leaf), x)
#define node(lhs, data, rhs) ML99_choice(v(node), lhs, data, rhs)

#define sumTree(tree)                     ML99_match(tree, v(sumTree_))
#define sumTree_leaf_IMPL(x)              v(x)
#define sumTree_node_IMPL(lhs, data, rhs) ML99_add3(sumTree(v(lhs)), v(data), sumTree(v(rhs)))

/*
 *         4
 *        / \
 *       /   \
 *      /     \
 *     2       6
 *    / \     / \
 *   1   3   5   7
 */
#define TREE node(node(leaf(v(1)), v(2), leaf(v(3))), v(4), node(leaf(v(5)), v(6), leaf(v(7))))

ML99_ASSERT_EQ(sumTree(TREE), v(28));

int main(void) {}

Defines

ML99_choice(tag, ...)

Constructs an instance of a choice type.

Examples

See examples/binary_tree.c.

Note

Specify ~ if you do not want to supply data; then, to match it, write a _ parameter to ignore.

ML99_choiceTag(choice)

Evaluates to the tag of choice.

This macro is essentially the same as ML99_tupleGet(0).

Examples

#include <metalang99/choice.h>

// foo
ML99_choiceTag(ML99_choice(v(foo), v(1, 2, 3)))

ML99_choiceData(choice)

Evaluates to the data of choice.

This macro is essentially the same as ML99_tupleTail.

Examples

#include <metalang99/choice.h>

// 1, 2, 3
ML99_choiceData(ML99_choice(v(foo), v(1, 2, 3)))

ML99_match(choice, matcher)

Matches the instance choice of a choice type.

This macro results in ML99_call(ML99_cat(matcher, ML99_choiceTag(choice)), <choice data>).

Examples

See examples/binary_tree.c.

ML99_matchWithArgs(choice, matcher, ...)

The same as ML99_match but supplies additional arguments to all branches.

This macro results in ML99_call(ML99_cat(matcher, ML99_choiceTag(choice)), <choice data>, args...).

Examples

#include <metalang99/choice.h>

#define MATCH_A_IMPL(x, y, z) v(x ~ y ~ z)

// 123 ~ 456 ~ 789
ML99_matchWithArgs(ML99_choice(v(A), v(123)), v(MATCH_), v(456, 789))

ML99_CHOICE(tag, ...)
ML99_CHOICE_TAG(choice)
ML99_CHOICE_DATA(choice)