The actual match function.
The value returned from the handler that matches the currently-held type.
Sometimes, implicit conversions may cause a handler to match more types than intended. The example below shows two solutions to this problem.
alias Number = SumType!(double, int); Number x; // Problem: because int implicitly converts to double, the double // handler is used for both types, and the int handler never matches. assert(!__traits(compiles, x.match!((double d) => "got double", (int n) => "got int"))); // Solution 1: put the handler for the "more specialized" type (in this // case, int) before the handler for the type it converts to. assert(__traits(compiles, x.match!((int n) => "got int", (double d) => "got double"))); // Solution 2: use a template that only accepts the exact type it's // supposed to match, instead of any type that implicitly converts to it. alias exactly(T, alias fun) = function(arg) { static assert(is(typeof(arg) == T)); return fun(arg); }; // Now, even if we put the double handler first, it will only be used for // doubles, not ints. assert(__traits(compiles, x.match!(exactly!(double, d => "got double"), exactly!(int, n => "got int"))));
std.variant.visit
Calls a type-appropriate function with the value held in a SumType.
For each possible type the SumType can hold, the given handlers are checked, in order, to see whether they accept a single argument of that type. The first one that does is chosen as the match for that type. (Note that the first match may not always be the most exact match. See "Avoiding unintentional matches" for one common pitfall.)
Every type must have a matching handler, and every handler must match at least one type. This is enforced at compile time.
Handlers may be functions, delegates, or objects with opCall overloads. If a function with more than one overload is given as a handler, all of the overloads are considered as potential matches.
Templated handlers are also accepted, and will match any type for which they can be implicitly instantiated. See "Introspection-based matching" for an example of templated handler usage.