Units of Measure in F#: Part Two, Unit Conversions

re: Units of Measure in F#: Part Two, Unit Conversions

alexey_r — Tue, 02 Sep 2008 16:59:20 GMT

A great feature!

How do unitful quantities look to the CLR? Something like

[Measure]

struct Second : IMeasure {}

and

[Measure]

struct Mult<M1,M2> : IMeasure where M1 : IMeasure where M2 : IMeasure {}

re: Units of Measure in F#: Part Two, Unit Conversions

wil2200 — Tue, 02 Sep 2008 22:00:52 GMT

Great post. However, I am having issues running the "gravitionalForce" portion of it.

I keep getting an error saying that "error FS0039: The namespace or module 'SI' is not defined." even though I referenced it in both ways (I also have powerpack added to the project):

open SI and/or

open Microsoft.FSharp.Math.SI

any thoughts on this?

Thanks

~sparky

re: Units of Measure in F#: Part Two, Unit Conversions

andrewkennedy — Tue, 02 Sep 2008 23:26:48 GMT

alexey_r:

Units actually don't get seen at all by the CLR - they are "erased". They are a purely static phenomenon, which means that the performance of code with units is no worse than the performance of the code with the units removed. You can think of them as a "refinement" of usual .NET types, seen only by F#. Of course this has some downsides (no runtime inspection of units) but our experience so far has been that they are in any case extremely useful in practice. And interestingly, all the "classical" functional languages (Haskell, Caml, OCaml, Standard ML) have purely static type systems.

sparky: did you include

open Microsoft.FSharp.Math

? This has PhysicalConstants as a submodule.

- Andrew.

re: Units of Measure in F#: Part Two, Unit Conversions

WillSmith — Wed, 03 Sep 2008 01:29:52 GMT

Would be nice to have unit of measure support as a general class library in the .net framework. Can you specify the scale and precision as well?

re: Units of Measure in F#: Part Two, Unit Conversions

aatreya — Wed, 03 Sep 2008 03:04:22 GMT

Andrew,

How can I get the following to compile?:

[<Measure>]

type km =

static member toM = 1.0/1000.0<m/km>

[<Measure>]

type m =

static member toKm = 1000.0<km/m>

Seems like there's a fundamental limitation I'm running up against... or hopefully I'm just missing something. The definition of m requires the definition of km, and vice versa.

re: Units of Measure in F#: Part Two, Unit Conversions

andrewkennedy — Wed, 03 Sep 2008 03:13:19 GMT

You can define mutually recursive measures using "and" to connect them and placing the Measure attribute immediately before the name of the measure:

type [<Measure>] km =

static member toM = 1.0/1000.0<m/km>

and [<Measure>] m =

static member toKm = 1000.0<km/m>

re: Units of Measure in F#: Part Two, Unit Conversions

aatreya — Wed, 03 Sep 2008 03:24:03 GMT

Wonderful - thanks! I'm so excited that a practical programming language finally has units-of-measure capability. Nice work.

re: Units of Measure in F#: Part Two, Unit Conversions

sparky00 — Wed, 03 Sep 2008 04:38:27 GMT

This is incredibly annoying.

[1] Reference FSharp Powerpack.dll - check

[2] open Microsoft.FSharp.Math - Alt+Enter -> FSI pass

[3] open SI - Alt+Enter -> FSI FAIL [error FS0039: The namespace or module 'SI' is not defined.]

doing something as simple as:

#light

open Microsoft.FSharp.Math

open SI

let distance (d:float<m>) = d

does not work -> anyone else has this problem?

The VS IntelliSense and the documentation says that the SI module is installed, yet when I open and try to use it, nothing works.

It seems to me that the problem is with the FSI and interpreting namespaces/modules but if anyone can give me some insights, that will really help!

re: Units of Measure in F#: Part Two, Unit Conversions

int19h — Wed, 03 Sep 2008 09:23:45 GMT

> As far as F# is concerned, ft and m have nothing to do with each other. It's up to you, the programmer, to define appropriate conversion factors.

This is mildly annoying. There's no reason why feet cannot be converted to meters and back automatically (it's not a narrowing conversion, so no opportunity for data loss), and even less for not being able to convert between m and km. It would seem to me that boost::units approach, which differentiates between abstract measures (duration, distance etc), and specific units of measurement, is more convenient.

re: Units of Measure in F#: Part Two, Unit Conversions

davidacoder — Wed, 03 Sep 2008 18:04:37 GMT

This whole thing is seriously cool. Just today we stubmled again with unit mix up in our simulation code.

But then, my most urgent question would be whether there are any plans to support this in other languages as well? F# is nice, but we have a large investment in C# and really wouldn't want to move to F# just for the unit stuff... Any ideas on that?

FIXED: error FS0039: The namespace or module 'SI' is not defined.

sparky00 — Wed, 03 Sep 2008 22:24:05 GMT

I finally got this to work and I realized where the problem was. I was using the default F# project and .fs file to type up all the code in but when I Alt+Enter and send to FSI, that is where the problem started.

Even though I added the reference via "Add Reference", I still get the error because FSI is not aware of project settings and all that. So in addition, I had to do #r "FSharp.PowerPack.dll" in the FSI window first, then Alt+Enter the rest of the code to it. Works now.

Thanks for the help.

~sparky

re: Units of Measure in F#: Part Two, Unit Conversions

int19h — Thu, 04 Sep 2008 09:23:55 GMT

It would seem to be that Microsoft is deliberately pushing for F# for scientific computing now, while C# is your typical "general-purpose" (read: system & LOB) language. C# 4.0 is expected to have 4 new language features, one of them generic variance - I wouldn't expect to see units among the other three. Nor anytime soon - DbC, for example, would probably be higher-priority for C#.

re: Units of Measure in F#: Part Two, Unit Conversions

yemi — Fri, 05 Sep 2008 01:30:42 GMT

Hello,

One thing that I stumbled upon. The value conversion on the types get masked when you set a literal with the same name as a type. In FSI:

[<Measure>] type km = static member pM = 1000.0<km/m>;;

[<Measure>] type m = static member pK = 0.001<m/km>;;

let km = 1.0<km>;;

km;;

m.pK;;

km.pM;;

The last line errors saying it can't find pM. Good day.

Yemi Bedu

re: Units of Measure in F#: Part Two, Unit Conversions

michen — Fri, 05 Sep 2008 07:39:10 GMT

Does F# support units with different zero values, e.g. Kelvin and Celsius? What I mean is that conversion of 100C to K depends on whether 100C in this case means temperature of boiling water (then the result is 373.15K) or difference between boiling water temperature and freezing water temperature (in this case the answer is 100K).

One can write two converters (say C.AbsToK and C.DeltaToK), but then it is user's responsibility to do it right - F# will do no checking.

Or maybe one should define two types of K and C units - absolute and delta, with rules like AbsX-AbsX=DeltaX, AbsX+DeltaX=AbsX, AbsX+AbsX=illegal, and so on?

Kelvin vs Celsius

andrewkennedy — Fri, 05 Sep 2008 18:38:27 GMT

Good point regarding units with different zero values. The units supported by F# are those for which multiplication makes sense, and Celsius doesn't fit this model. It makes no sense to double an absolute temperature measured in degrees Celsius. Whereas for Kelvin, this is fine.

A similar situation arises with absolute times versus time periods - it would indeed be nice to permit subtraction of absolute times, and rule out addition of absolute times, but permit either operation on time *periods*. One can do this by defining separate *types*, a little like DateTime and TimeSpan from the .NET framework.

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