On Tuesday, 7 June 2016 at 03:55:03 UTC, none wrote:
import std.algorithm.iteration : map;
import std.algorithm : castSwitch;
import std.format : format;
class A { int value; this(int value) { this.value = value; }}
interface I { }
class B : I { }
Object[] arr = [new A(5), new B(), null];
auto results = arr.map!(castSwitch!(
(A a) => "class A with a value of %d".format(a.value),
(I i) => "derived from I",
(B b) => "class B",
() => "null reference",
))();
The codes get nonsense error like:
/home/bthach/dlang/dmd-2.071.0/linux/bin64/../../src/phobos/std/algorithm/comparison.d(277):
Error: no property 'format' for type 'string'
Note the location of the error: it is in Phobos, not in your
code. It's a bug, it should actually print a valid error message.
Filed here:
https://issues.dlang.org/show_bug.cgi?id=16135
It has nothing to do with the fact that you're using `format`,
that's just coincidence.
But when I changed the order:
[...]
(B b) => "class B",
(I i) => "derived from I",
Suprisingly, it works.
`castSwitch` tries the types you pass it in the order you
specify. If you pass `I` first, the `B` case will never be
matched because it is "shadowed" by the former (every `B` is also
an `I`). `castSwitch` detects this situation and wants to print a
message to warn you. However, someone forgot to import
`std.format` (which is used for formatting the error message),
resulting in the error you observed.
