[Issue 12100] New: __GENTYPE to generate ever different types
d-bugmail at puremagic.com
d-bugmail at puremagic.com
Fri Feb 7 10:35:53 PST 2014
https://d.puremagic.com/issues/show_bug.cgi?id=12100
Summary: __GENTYPE to generate ever different types
Product: D
Version: D2
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P2
Component: DMD
AssignedTo: nobody at puremagic.com
ReportedBy: bearophile_hugs at eml.cc
--- Comment #0 from bearophile_hugs at eml.cc 2014-02-07 10:35:50 PST ---
Currently in std.typecons there is Typedef, that the docs describe with:
template Typedef(T)
struct Typedef(T, T init, string cookie = null);
The cookie is needed when you want to create different types based on the same
type:
alias MyInt1 = Typedef!(int, 0, "A");
alias MyInt2 = Typedef!(int, 0, "B");
To remove the need of the cookie I suggest to add to D something that could be
named __GENTYPE, that every time creates a new type:
struct Typedef(T, T init=T.init, NT=__GENTYPE);
So every instantiation of Typedef is different:
alias MyInt1 = Typedef!int;
alias MyInt2 = Typedef!int;
static assert (!is(MyInt1 == MyInt2));
A __GENTYPE is also a very simple way to implement the "Path Dependent Types"
of the Scala language. Here in Scala b1 and b2 have different type:
http://stackoverflow.com/questions/2693067/what-is-meant-by-scalas-path-dependent-types
class A {
class B {
}
}
var a1 = new A;
var a2 = new A;
var b1 = a1.new B;
var b2 = a2.new B;
Parametrizing the class A with an unused type, and using __GENTYPE inside a
helper function that instantiates A, you can have the same feature in D:
class A(Ghost) {
class B {
}
}
The disadvantage is that you have template bloat, so here it becomes useful an
idea I suggested to reduce template bloat, templated():
http://www.digitalmars.com/d/archives/digitalmars/D/templated_126655.html
The name __GENTYPE comes from the Lisp gensym that's used to generate new names
to be used in macros.
http://en.wikipedia.org/wiki/Hygienic_macro#Strategies_used_in_languages_that_lack_hygienic_macros
http://www.lispworks.com/documentation/HyperSpec/Body/f_gensym.htm#gensym
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