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77 lines
2 KiB
Text
77 lines
2 KiB
Text
New module: `std.sumtype`
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The [`sumtype` package](https://code.dlang.org/packages/sumtype) from
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code.dlang.org has been added to the standard library as `std.sumtype`.
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It provides `SumType`, a generic discriminated union implementation that uses
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[design-by-introspection](https://www.youtube.com/watch?v=HdzwvY8Mo-w) to
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generate safe and efficient code, and is intended to serve as a replacement for
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the legacy `std.variant.Algebraic`.
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Features of `SumType` include:
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* Pattern matching.
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* Support for self-referential types.
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* Full compatibility with `pure`, `@safe`, `@nogc`, `nothrow`, and `scope`.
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* No dependency on runtime type information (`TypeInfo`).
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* Compatibility with BetterC.
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Example usage:
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---
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import std.sumtype;
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import std.math : isClose;
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struct Fahrenheit { double degrees; }
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struct Celsius { double degrees; }
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struct Kelvin { double degrees; }
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alias Temperature = SumType!(Fahrenheit, Celsius, Kelvin);
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// Construct from any of the member types.
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Temperature t1 = Fahrenheit(98.6);
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Temperature t2 = Celsius(100);
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Temperature t3 = Kelvin(273);
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// Use pattern matching to access the value.
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Fahrenheit toFahrenheit(Temperature t)
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{
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return Fahrenheit(
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t.match!(
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(Fahrenheit f) => f.degrees,
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(Celsius c) => c.degrees * 9.0/5 + 32,
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(Kelvin k) => k.degrees * 9.0/5 - 459.4
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)
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);
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}
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assert(toFahrenheit(t1).degrees.isClose(98.6));
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assert(toFahrenheit(t2).degrees.isClose(212));
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assert(toFahrenheit(t3).degrees.isClose(32));
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// Use ref to modify the value in place.
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void freeze(ref Temperature t)
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{
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t.match!(
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(ref Fahrenheit f) => f.degrees = 32,
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(ref Celsius c) => c.degrees = 0,
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(ref Kelvin k) => k.degrees = 273
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);
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}
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freeze(t1);
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assert(toFahrenheit(t1).degrees.isClose(32));
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// Use a catch-all handler to give a default result.
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bool isFahrenheit(Temperature t)
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{
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return t.match!(
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(Fahrenheit f) => true,
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_ => false
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);
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}
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assert(isFahrenheit(t1));
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assert(!isFahrenheit(t2));
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assert(!isFahrenheit(t3));
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---
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