Fable: Unsound decimal behavior

[toburger]

Description

I know decimals in fable aren't really decimals, but this was quite a surprise for me:

Repro code

let items = [
    290.8M
    290.8M
    337.12M
    6.08M
   -924.8M
]

if List.sum items = 0M then
    // do something if sum is zero
else
    // do something else

Link to Fable REPL

Expected and actual results

I was expecting that the sum of the following values to be 0.
The actual value is 1.1368683772161603e-13.

This is quite unexpected for an F# developer where one of the language slogans is robustness.
The result is totally expected for floats, but it is a bug for decimals.

The following solutions would help avoid imprecise decimal values and gain trust in Fable:

• Disallow decimals. This sounds very radical, but since there is no equivalent replacement for decimals it is the best to give a compiler error before such kind of bugs can happen.
• Same as above, but give a warning.
• Replace decimals with something provided by Fable.

[alfonsogarciacaro]

We needed decimals to compile the F# compiler with Fable, compiling them as decimals was only a temporary solution but it's been working so we haven't "fixed" it yet. We do have a warning when explicitly converting floats to decimals but we don't for every use of decimals to prevent polluting the logs.

In any case I do agree we should implement decimals the correct way (as we do for int64), I wanted to do it for Fable 2 but other tasks were given higher priority at the end. The main part should be to port this file to F#, any volunteers?

[alfonsogarciacaro]

Or maybe using a tool to directly translate the C# code to JS? Or is there already a JS lib which is easy to integrate and has the proper semantics? Pinging @ncave for ideas.

[ncave]

@alfonsogarciacaro A lot of things can work, e.g. Bridge.NET uses decimal.js, or we can use F#'s BigRational, or port from C#, etc.

[ncave]

@alfonsogarciacaro Or, more appropriately, decimal.js-light, which is the smaller version (without the trig. functions), or the even smaller one, big.js.

[toburger]

Hmm, maybe this is something I could contribute to Fable... 🤔

Maybe we could port the implementation from elm: https://github.com/torreyatcitty/the-best-decimal/blob/1.0.0/src/Decimal.elm

This solution is based on BigInt (BigRational).
Is BigInt implemented in Fable? Fable REPL doesn't seem to be aware of big integers?
_Anyways, an implementation of BigInt can be found here: https://github.com/cmditch/elm-bigint/blob/master/src/BigInt.elm_

What do you think, should we port elm's (functional) implementation or the C# (imperative?) code?

Slightly off topic:
There is at the moment an ongoing discussion at hackernews about how to deal with monetary values in JavaScript: https://frontstuff.io/how-to-handle-monetary-values-in-javascript and https://news.ycombinator.com/item?id=18334865
I think with a correct implementation of decimals we can do the right thing out of the box in Fable! 😄

[toburger]

Correction:

BigInt is implemented, but the literal doesn't seem to work:

• bigint.Parse "12345678912345" works and gives 12345678912345
• 12345678912345I gives 1942903641

[ncave]

@toburger As mentioned above, there is F#'s BigRational (based on F# BigInt which Fable already has, so nothing to port), but IMO we should be using the best performing solution, since performance is a feature.

[ncave]

@toburger Thanks for the report, I opened an issue #1623 for it.

[alfonsogarciacaro]

Thanks for letting me know about BigRational, @ncave! This probably would be the easiest way to go as we already have BigInt (though with some problems in the REPL as @toburger points out) and is in line with our efforts to bring more F# modules to fable-core.

However, you're also right that this can have a performance hit (I guess especially in the REPL). Porting System.Decimal from C# is probably not an option as I just realized it uses unsafe code. So the main alternative would be to use decimal.js-light. I assume we would have to restructure the code as we did for long.js if we want to take advantage of tree shaking and have a better integration in the replacements module. It's a bit of work but I can do it (I'm only concerned about introducing bugs, we would need to port some tests from dotnet).

[toburger]

Fantastic work on the decimal support! Thank you! 👏

I have a little quirk with toLocaleString, which doesn't work with decimals anymore.

let formatAsCurrency (d: decimal) =
    d?toLocaleString(locale, createObj [
        "style" ==> "currency"
        "currency" ==> "EUR"
    ])

This outputs (now) the plain value 3305.45 for instance...

The workaround is to converts it to a float before calling toLocaleString.

let formatAsCurrency (d: decimal) =
    (float d)?toLocaleString(locale, createObj [
        "style" ==> "currency"
        "currency" ==> "EUR"
    ])

which outputs the desired value 3.305,45 €.

[alfonsogarciacaro]

Wow, didn't know about the toLocaleString API, thanks for pointing it out! 😅 Well, it makes sense it doesn't work with decimals now. I'm reluctant to add support to System.Globalization in Fable because that'll be a lot of work, but it could be nice to add number formatting to Fable.Powerpack with a nice API. Would you like to PR it?

[toburger]

That would be really cool to have!

I've created some helper functions for my app but it would be really nice to have a nice statically typed API for formatting.

There exist four toLocaleString implementations for the following types:

• Number.toLocaleString
• Date.toLocaleString
• Array.toLocalString
• Object.toLocaleString

I think it would make sense to implement them for numbers and dates.
Although I have no idea how a nice API could look like. Maybe something like this?

open Fable.PowerPack.Locale

Number.currency Euro // discriminated union?
|> Number.format 20.10 // use browsers locale

Number.percent
|> Number.withMinimumFractionDigits 2
|> Number.formatAs "de-DE" 0.10 // use specific locale

Date.date
|> Date.withMonthAs Long
|> Date.format (System.DateTime.Now)

[alfonsogarciacaro]

Looks good! @MangelMaxime added the date formatters to Fable.PowerPack, what do you think?