On Wednesday, 1 April 2015 at 17:51:40 UTC, John Colvin wrote:
On Wednesday, 1 April 2015 at 17:03:34 UTC, Dennis Ritchie
wrote:
On Wednesday, 1 April 2015 at 15:22:10 UTC, John Colvin wrote:
Compile Time Function Evaluation (CTFE) is a very powerful
tool
to avoid having to enter in to all that C++ style mess.
Yes, CTFE in D really cool. Thanks.
I need to implement arithmetic (addition / subtraction) only
use the type system.
That is, such a design does not suit me:
enum Add(Args...) = Args[0] + Args[1];
enum Inc(Args...) = Args[0] + 1;
Don't really see the point. Here's a neat thing that's
definitely cheating because although it stores the results in
the type system, the arithmetic is done in constant-folding:
struct Integer(int a){}
template Value(T)
{
static if (is(T == Integer!a, int a))
enum Value = a;
else static assert(false, "Can't get Value for " ~
T.stringof);
}
alias Inc(T) = Integer!(Value!T + 1);
But if you really insist on it being all type-system (until you
wan't the answer of course):
struct Zero{}
template Succ(T)
{
static if (is(T == Pred!A, A))
alias Succ = A;
else
struct Succ{}
}
template Pred(T)
{
static if (is(T == Succ!A, A))
alias Pred = A;
else
struct Pred{}
}
enum isPositive(T) = is(T == Succ!A, A);
enum isNegative(T) = is(T == Pred!A, A);
enum isZero(T) = is(T == Zero);
template Add(A, B)
{
static if (isZero!B)
alias Add = A;
else static if (isPositive!B)
alias Add = Add!(Succ!A, Pred!B);
else
alias Add = Add!(Pred!A, Succ!B);
}
template Value(T, int seed = 0)
{
static if (isZero!T)
enum Value = seed;
else static if (isPositive!T)
enum Value = Value!(Pred!T, seed+1);
else
enum Value = Value!(Succ!T, seed-1);
}
unittest
{
alias One = Succ!Zero;
alias Two = Succ!One;
alias MinusThree = Pred!(Pred!(Pred!Zero));
static assert (Value!Zero == 0);
static assert (Value!One == 1);
static assert (Value!Two == 2);
static assert (Value!MinusThree == -3);
static assert (Value!(Add!(One, MinusThree)) == -2);
static assert (Value!(Add!(One, Two)) == 3);
static assert (is(Add!(Add!(One, Two), MinusThree) == Zero));
}
If the is() expressions are confusing you, in this case they work
like this;
is (T == B!A, A)
means
is T the same type as B instantiated with A, for some type A?
or more succinctly
is T an instantiation of B?
See http://dlang.org/expression.html#IsExpression, it's quite
reminiscent of some mathematical set notation.
