Code shared from run.dlang.io.

main.d

import std.stdio;

mixin template Enforcement(T) {
    private bool _ensured;
    private Throwable _throwable;
    
    ~this() {
        if (this.hasThrowable) {
            throw _throwable;
        }
    }
    
    auto enforce(bool ensured) {
        _ensured = ensured;
        
        return this;
    }
    
    auto releaseThrowable() {
        scope(exit) _throwable = null;
        
        return _throwable;
    }
    
    bool hasThrowable() const {
        return _throwable !is null;
    }
    
	bool isEnsured() const {
    	return !_ensured;
    }
    
    auto orThrow(string message) {
        if (!this.isEnsured) {
        	this.orThrow(new Exception(message));
        }
        
        return this;
    }
    
    auto orThrow(lazy Throwable throwable) {
        if (!this.isEnsured) {
            if (this.hasThrowable) {
            	throwable.next(_throwable);
            }
            
            _throwable = throwable;
        }
        
        return this;
    }
}

mixin template EnsuranceImpl(T)
{
    private T _value;

    this()(Ensurance!T ensurance) {
        _value = ensurance.get;
        _ensured = ensurance.isEnsured;
        _throwable = ensurance.releaseThrowable;
    }
    
    mixin Enforcement!T;
}

struct StringEnsurance {
    mixin EnsuranceImpl!string;
    
    auto hasLength(size_t length) {
        return this.enforce(_value.length == length);
    }
}

struct BooleanEnsurance {
    mixin EnsuranceImpl!bool;
}

struct Ensurance(T) {
    import std.traits: isSomeString, isArray, isScalarType, isNumeric, isBoolean;
    
    private T _value;
    
    mixin Enforcement!T;
    
    this(T value) {
        _value = value;
    }
    
    auto get() {
        return _value;
    }
    
    auto isString() {
        return StringEnsurance(this.enforce(isSomeString!T));
    }
    
    auto isArray() const {
        return isArray!T;
    }
    
    auto isScalar() {
        return isScalarType!T;
    }
    
    auto isNumeric() {
        return isNumeric!T;
    }
    
    auto isBool() {
        return isBoolean!T;
    }
}

auto ensure(T)(T value) {
    return Ensurance!T(value);
}

void main() {
    writeln(ensure("Hello D").isString.hasLength(42));
}