As a learning exercise, I'm writing a simple AST evaluator for
arithmetic expressions in D. (See
https://gist.github.com/ckirkendall/2934374) In order to get a
feel for functional programming in D, I decided to try and
implement my solution in a functional style, using algebraic data
types and pure functions.
Here's the code:
import std.variant;
import std.typecons;
enum Op { Plus, Minus, Times, Div }
alias Expr = Algebraic!(
double,
string,
Tuple!(Op, "op", This*, "lhs", This*, "rhs")
);
double eval(Expr expr, double[string] env) pure {
return expr.visit!(
(double x) pure => x,
(string v) pure => env[v],
(Tuple!(Op, "op", Expr*, "lhs", Expr*, "rhs") bop) pure {
final switch (bop.op) with (Op) {
case Plus:
return eval(*bop.lhs, env) + eval(*bop.rhs, env);
break;
case Minus:
return eval(*bop.lhs, env) - eval(*bop.rhs, env);
break;
case Times:
return eval(*bop.lhs, env) * eval(*bop.rhs, env);
break;
case Div:
return eval(*bop.lhs, env) / eval(*bop.rhs, env);
break;
}
}
)();
}
When I compile this (using DMD 2.069 on Debian Linux), I get an
error saying that I can't call visit from a pure function. This
is surprising, since all visit does (in theory) is call the
provided functions, and all of _them_ are pure.
My question is, is this an unavoidable limitation of visit's
implementation, or is there a way to work around this?