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/*
* Let us have a simple arithmetical language, consisting of:
*
* 1) Constants of double;
* 2) Expressions: either expr1 (+ | - | * | /) expr2 or a constant.
*
*
* A sum type can be used to represent the hierarchical structure of ASTs, and pattern matching is a
* technique to evaluate them.
*/
#include <datatype99.h>
#include <stdio.h>
// clang-format off
datatype(
Expr,
(Const, double),
(Add, Expr *, Expr *),
(Sub, Expr *, Expr *),
(Mul, Expr *, Expr *),
(Div, Expr *, Expr *)
);
// clang-format on
double eval(const Expr *expr) {
match(*expr) {
of(Const, number) return *number;
of(Add, lhs, rhs) return eval(*lhs) + eval(*rhs);
of(Sub, lhs, rhs) return eval(*lhs) - eval(*rhs);
of(Mul, lhs, rhs) return eval(*lhs) * eval(*rhs);
of(Div, lhs, rhs) return eval(*lhs) / eval(*rhs);
}
// Invalid input (no such variant).
return -1;
}
#define EXPR(expr) ((Expr *)(Expr[]){expr})
#define OP(lhs, op, rhs) op(EXPR(lhs), EXPR(rhs))
int main(void) {
// 53 + ((155 / 5) - 113)
Expr expr = OP(Const(53), Add, OP(OP(Const(155), Div, Const(5)), Sub, Const(113)));
/*
* Output:
* -29.000000
*/
printf("%f\n", eval(&expr));
return 0;
}
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