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digitalmars.D.learn - Making pure functions get dirty (D 2.0)

reply Burton Radons <burton.radons gmail.com> writes:
I'm writing a general conversion template function a la:

  pure T convert (T, U) (const (U) value);

Sweet, and really handy for template errors because you can tell the user which
number input it is that angered it. The problem is that if you're converting
int to string there's allocations there that doesn't have to be there for many
purposes (also I'm using it for error formatting, which isn't so good if what
I'm telling the user is that there's no memory for allocations). So I thought
I'd make it work like this:
  // May call write multiple times for arrays, each time builds onto the array.
  void convertInto (T, U, alias write) (const (T) value)
    write ("You've bespoiled my honour, cur!\n");

  void convertWith (T, U) (const (U) value, void delegate (T) write)
    convertInto! (T, U, write) (value);

  import std.stdio;
  void main ()
    void write (string text) { writef (text); }
    convertWith! (string, string) ("Let's you and I spend a few moments alone
without a chaperone, baby.\n", &write);

Strangely enough this compiles but it doesn't actually DO anything (everything
works, it's just that convertInto is not called at all); just taking the pure
off fixes it. Obviously it shouldn't work, but what would I do to make an
conditionally-pure function like this without code replication or mixin madness?

It feels like I'm missing some kind of "third way".
Feb 20 2009
next sibling parent Burton Radons <burton.radons gmail.com> writes:
Burton Radons Wrote:

   void convertInto (T, U, alias write) (const (T) value)
This should read "pure void". Everything I said about its behaviour is correct for my experiences.
Feb 20 2009
prev sibling parent Burton Radons <burton.radons gmail.com> writes:
Oh I see what's going on. pure functions get funky processing; if you don't
actually use their return values they're not even compiled. Once you actually
take the return value it'll complain about it whether it's a pure inner
function in a pure outer function or anything else unless if it's a pure outer
function which is perhaps the most useless thing you could have in this context.
Feb 20 2009