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.
>>>
>>>

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