I used this in a small unit library (partially accessible on
github),
to obtain code like:
auto distance = 100.km;
auto speed = 130.km/h; // division works, too.
auto timeToDestination = (distance/speed).hour; //
distance/speed
gives seconds => transformed in hours.
It was a nice exercise in using UFCS and mixins to create your
own
unit library (not only IS, but ay kind of unit library).
And, you know what? I *never* used it after coding it. These
examples
are cute, they make for nice blog posts for F#, but the
real-world
usage is dubious to me (I know they were space-programs
crashes)
I quite like the implicit message in units: use the type
system to
help you catch errors are compile-time. Add to that a nice
syntax and
a showcase for D's generational capabilities and it's quite
nice.
But, to my eyes, it's but a toy.
I wouldn't read too much into it. You're a library author, not
(I assume) a scientific computing guy. So beyond playing with a
few examples, your work on this library is done - you wouldn't
be a client of it for the simple reason you don't intensively
work with kilometers, speeds, dollars, and such. It's possible
that a good and usable library of units could add value to a
category of users.
IMO, you don't need to be a scientific computing guy to find unit
checking useful, since almost any number conceptually has a unit
on it. I would ask any programmer, how often do you accidentally
use a measurement of 'bytes' where 'dwords' were expected, or use
a variable as an array index when it was actually something
totally different?
However, unit checking cannot be done satisfactorially in a
library; it has two main problems when provided that way:
1. It's too bulky (too much syntax required, as units have to be
spelled out constantly)
2. Values with traditionally-typed units don't interoperate with
existing libraries, including very simple functions such as
int abs(int x) { return x > 0 ? x : -x; }
int square(int x) { return x*x; }
You can define an inplicit conversion from e.g. 'Unit!"pixels"'
to 'int' but then you'll need to manually cast it back, and the
compiler can't check your cast to make sure it's correct.
IMO, solving these two problems requires a parallel type system
to infer unit relationships automatically, either with direct
language support, or a separate analysis tool that uses the
compiler as a service (currently not possible with D).
