struct Node {
struct BinaryTree *l;
int x;
struct BinaryTree *r;
};
Sumtype(
BinaryTree,
(int, leaf),
(struct Node, node)
)
int sum(struct BinaryTree *tree) {
match(struct BinaryTree, tree) {
let(int, leaf) return *leaf;
let(struct Node, node) return sum(node->l) + node->x + sum(node->r);
}
//if you are using GNU extensions or C2x, the types can be inferred!
#if Sumtype_typeinference
match_tree(tree) {
let(leaf) return *leaf;
let(node) return sum(node->l) + node->x + sum(node->r);
}
#endif
// Unreachable
return 0;
}Safe, intuitive sum types with exhaustive pattern matching in a single 100 line header, pure C99.
Inspired by datatype99, but consisting of one small standard-conforming C99 macro-only header that is fast to compile.
Just include the header file sumtype.h.
The provided macros are a syntactic sugar over tagged unios. The above code is equivalent to:
struct BinaryTree {
enum { Leaf, Node } tag;
union {
int leaf;
struct Node node;
} data;
};
int sum(struct BinaryTree *tree) {
switch (tree->tag) {
case Leaf:
return tree->data.leaf;
case Node:
return sum(tree->data.node.lhs) + tree->data.node.x + sum(tree->data.node.rhs);
}
// Unreachable.
return 0;
}The data type is generated with the Sumtype macro, which takes the name of the resulting
struct and and list of pairs of types and names of the variants, all separated by commas.
Sumtype(Haha, (char *, name), (uint16_t, id), (uint32_t, var))You can pattern-match over the instances of this type. The match macro takes a type
and an expression evaluating to a pointer to a value of this type.
Then, for each variant, the let macro takes a type and name of the given variant.
If the value being matched is of this variant, the let macro generates a binding
to a pointer to a value of this type with the provided name, which you may use in the
following statement.
match(struct Haha, &something) {
let(char *, name) {
printf("%s\n", *name);
}
let(uint16_t, id) {
printf("%d\n", *id);
}
otherwise {
printf("var\n");
}
};otherwise matches all variants.
For each variant, a constructor with the name Typename_variantname is generated.
struct BinaryTree leaf5 = BinaryTree_leaf(5);If you want to just test for one variant, the iflet macro is provided, which takes
the sum type, the type and the name of the variant being matched and an expression that evaluates
to a pointer of the sum type. It provides a binding analogical to let in match.
iflet(struct BinaryTree, int, leaf, &tree) {
printf("A leaf with the value %d\n", *leaf);
} else {
printf("Not a leaf\n");
}
#if Sumtype_typeinference
iflet_t(leaf, &tree) {
printf("A leaf with the value %d\n", *leaf);
} else {
printf("Not a leaf\n");
}
#endifFinally, the MATCHES macro, taking a name of a variant and an expression, tests whether
the value of the expression is of the given variant.
datatype99 is written using an impressive functional language built atop of preprocessor
macros, Metalang99. While this is a powerful
language and the resulting macros are more flexible, there are several reasons why
sumtype.h may be considered:
- It is a 100 line single header that you can just copy into your project and even quickly understand and modify to your liking.
- The compile times are much faster.
- It allows matching against
constpointers. - It doesn't pollute the namespace with any typedefs. It only expands to a struct with the provided name (contrary to a separate new type for all variants).
<sumtype> ::= "Sumtype(" <ident> { "," "(" <type>, <ident> ")" }+ ")" ;
<match> ::= "match(" <type> "," <expr> ") {" { <let> }* [ <otherwise> ] "}" ;
<of> ::= "let(" <type> "," <variant-name> ")" <stmt> ;
<otherwise> ::= "otherwise" <stmt> ;
<if-let> ::= "iflet(" <type> "," <type> "," <ident> "," <expr> ")" <stmt> ;
<matches> ::= "MATCHES(" <ident> "," <expr> ")" ;The code
Sumtype(A, (Type0, name0), ..., (TypeN, nameN))is expanded to
struct A {
enum {
SUMTYPE_TAG_name0,
...
SUMTYPE_TAG_nameN,
} tag;
union {
Type0 name0;
...
TypeN nameN;
} variant;
}
struct A A_name0(Type0 name0) {
return (struct A) {
.tag = SUMTYPE_TAG_name0,
.variant = {
.name0 = name0
}
}
}
...
struct A A_nameN(TypeN nameN) {
return (struct A) {
.tag = SUMTYPE_TAG_nameN,
.variant = {
.nameN = nameN
}
}
}match has the expected semantics: it sequentially tries to match the given instance of
a sum type against the given variants, and, if a match has succeeded, it executes the
corresponding statement and moves down to the next instruction
(match(val) { ... } next-instruction;). If all the matches have failed,
it executes the statement after otherwise and moves down to the next instruction.
A complete match construct results in a single C statement.
let accepts a type and a matched variant name as its arguments. If the variant is matched,
it binds a variable with the provided name to a pointer to the value of the provided type.
iflet tries to match the given instance of a sum type against the given variant,
and, if a match has succeeded, it executes the corresponding statement.
A complete iflet construct results in a single C statement.
MATCHES just tests an instance of a sum type for a given variant.
If the given instance corresponds to the given variant, it expands to truthfulness, otherwise it expands to falsehood.
The name of all the variants used, even in different types, need to be unique. Otherwise, the names fo their tags will collide.
break/continue inside a statement provided to let/iflet
but outside of any for/while loops in that statement just terminates the match/iflet
statement, but not any outside loop. Use a goto to break out of the outside loop.
Bindings introduced by let are mutable, unless you write const in front of the name
of the type. The compiler will warn you about this if you match against a const pointer.
Public domain or WTFPL, at your choice.