Btw, when using (:::) this way, it's nice to set the operator precedence to something lower than 9 so you can say `2 * 5 + 3 ::: radians` without having to put brackets around the numbers. I think `infix 5 :::` gives the right precedence; Looking at the core docs ( https://github.com/elm-lang/core/blob/master/src/Basics.elm ), (+) and (-) are 6, and (==), (/=), etc are 4.
On Friday, July 29, 2016 at 12:50:24 PM UTC-5, John Bugner wrote: > > I like the clever use of the (:::) function to make its function argument > (somewhat) look like a type. > > On Friday, July 29, 2016 at 11:48:26 AM UTC-5, Anton Lorenzen wrote: >> >> I just created a small library for this: >> https://github.com/anfelor/elm-units >> >> It allows for time to be used like this: >> Units.Time.every (30 ::: milliseconds) Tick >> >> Am Donnerstag, 28. Juli 2016 20:42:44 UTC+2 schrieb John Bugner: >>> >>> In the time library ( >>> package.elm-lang.org/packages/elm-lang/core/4.0.3/Time ), there are >>> many functions under the "Units" heading. Under the function `millisecond`, >>> it says: >>> >Units of time, making it easier to specify things like a half-second >>> (500 * millisecond) without remembering Elm’s underlying units of time. >>> I find this underwhelming, because nothing forces me to use these unit >>> functions. I could write `every 5 toMsg` and the compiler will not stop me. >>> How long of a time am I actually specifying? Probably 5 milliseconds, >>> because I infer that it's probably using the same unit as JavaScript, but I >>> can't know for sure. A beginner might think 5 seconds, because it's the >>> unit that has no prefixes, or 5 minutes if 5 minutes seems appropriate to >>> what the application does. This is very bad; There should never be doubt >>> about what unit is being used, and the compiler should enforce this unit >>> correctness. Currently, Elm can't do this, because `Time` is just an alias >>> for `Float`. >>> >>> To prevent this kind of error, I propose a new language construct that I >>> call a "unit type". It would have the following properties: >>> (1) A definition that would look (very roughly) like this: >>> ``` >>> type unit Time as Float >>> = Second >>> | Millisecond == Second 0.0001 >>> | Nanosecond == Millisecond 0.0001 >>> | Minute == Second 60 >>> | Hour == Minute 60 >>> | Day == Hour 24 >>> | Year == Day 365 >>> ``` >>> >>> The point of the definition is to: >>> (a) Provide an easy way to define different units that measure the same >>> thing (in this case, time, but you could do the same thing for >>> length/height/depth in either metric or US imperial units) as constructors. >>> (b) Tell how they are related to eachother. (A minute is 60 seconds, an >>> hour is 60 minutes, etc.) The compiler would check that all relations of >>> the constructors eventually flow to a single base unit (in this case, >>> `Second`). A cycle would be disallowed. >>> (c) Tell what type the unit is based on (in this case, `Float`). >>> (Perhaps non-number types would be disallowed.) >>> >>> (2) Given that `every` still has the same signature: `Time -> (Time -> >>> msg) -> Sub msg`, `every 5 toMsg` would cause a compile-time error, because >>> the types don't match; The function expects a number with a `Time` unit, >>> but receives `5`, which is just a raw number (a number without any unit). >>> (3) When writing a function that has a parameter of type `Time`, pattern >>> matching only matches the base unit constructor, not every constructor like >>> a normal (data/enum) type. The compiler would automatically convert the >>> other units to the base unit with the conversions that the programmer >>> provided in the definition. >>> (4) Comparison, addition, and subtraction would be automatically >>> implemented for the type, so two times of any combination of constructors >>> could be compared, added, or subtracted with ease. `(Minute 5) - (Second >>> 20) == (Millisecond 280000)` would "just work". "Time + Float" (or "Time + >>> Length") would cause a compile-time error. >>> (5) Perhaps compound unit types like "Time^2" would be supported, so a >>> "Time * Time" would yield "Time^2", "Time * Float" would yield "Time", and >>> "Time * Length" would yield just that: "Time * Length". ("Force" would be >>> an alias of "Mass * Length / (Second^2)".) >>> >>> I know that this is a very radical proposal, (I don't know of any >>> language that has a feature like this.) but I bring it up anyways, because >>> although it's been 18 years since the mars probe crashed because of a unit >>> error (one module assumed that the number it was getting was in metric >>> units, and another assumed that it was getting it in US imperial units) ( >>> https://en.wikipedia.org/wiki/Mars_Climate_Orbiter ), I'm amazed that >>> since then, programming languages have done nothing to prevent this kind of >>> error from happening again, besides just admonishing programmers to be >>> careful. (As if the NASA programmers at the time weren't already trying to!) >>> >>> Letting `5` be a legal unitless `Time` value is just as silly and >>> dangerous as letting `bool a = 2;` be a legal statement in C. data/enum >>> types prevent this from happening to `Bool` in Elm, and unit types could >>> prevent the same kind of thing from happening to `Time`. >>> >>> Questions, comments, related thoughts, etc are welcome. >>> >>> -- You received this message because you are subscribed to the Google Groups "Elm Discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to elm-discuss+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
