On Saturday, 13 September 2014 at 15:36:30 UTC, bearophile wrote:
This is a little Haskell program that uses the Maybe type
constructor:
foo :: Int -> Maybe Int
foo x | x < 10 = Just x
foo _ = Nothing
main = do
print $ foo 5
print $ foo 15
Its output:
Just 5
Nothing
This is a similar D program, that uses Nullable:
import std.stdio, std.typecons;
Nullable!int foo(int x) {
return (x < 10) ?
typeof(return)(x) :
typeof(return)();
}
void main() {
writeln(foo(5));
writeln(foo(15));
}
Its output:
5
core.exception.AssertError@C:\dmd2\src\phobos\std\typecons.d(1515):
Called `get' on null Nullable!int.
...
I think it's better for write(Nullable!int()) to not raise an
error, but to print something like a "<null>" etc.
A bigger problem is in the usage of Nullable. I'd like the D
type system to be modified and improved to support Nullables
with a nicer syntax.
Bye,
bearophile
I think Nullable should be completely replaced with an Option!T
type that is also a range, and one that also doesn't `alias this`
itself to its store. Then the above code would just print "[]"
for an empty range. I'm sure you remember this thread:
http://forum.dlang.org/thread/[email protected]
The problem is that Nullable internally has a `get` function that
checks if it is "null", and throw an error if so. It then does
`alias get this`, which means that in some contexts, operations
will be forwarded to `get`, and I guess one of those contexts is
when passing the Nullable to writeln. I've run into this problem
before, and I *thought* it had been fixed, but I guess not.
