Add range match support for comparable values in the IR

[AttilaMihaly]

The need to map ranges of numeric values to various categories comes up frequently in finance but there is no convenient way to represent those in Morphir. The closes you can get is an if-then-else chain:

if foo <= 6 then 1
else if foo <= 10 then 2
else foo * 2

While this is somewhat manageable it is not a direct representation of the business problem and it's easy to make mistakes which are difficult to spot. For example this one:

if foo <= 6 then 1
else if foo >= 10 then 2
else foo * 2

This is very confusing because the ranges are not in a sequential order in the code.

This pattern is so common though that I think it deserves direct language support and I think we can easily add it. To make this possible we just need a simple API in Elm:

rangeMatch foo Inclusive
        |> below 6 (always 1)
        |> below 10 (always 2)
        |> above ((*) 2)

There are a few advantages here:

• Less repetition (foo only appears once)
• Ranges are guaranteed to be sequential.
• Inclusive/exclusive range boundaries are explicit and applied consistently

Constraints on the structure of the range match will allow us to do more efficient optimizations in our backends and most importantly make it very easy to verify correctness (it simply amounts to checking if the boundaries are ordered correctly).

[AttilaMihaly]

Here's one potential implementation for the API used in the code above:

type RangeMatch a b
    = Next RangeType a
    | Done b


type RangeType
    = Inclusive
    | Exclusive


rangeMatch : comparable -> RangeType -> RangeMatch comparable a
rangeMatch subject rangeType =
    Next rangeType subject


below : comparable -> (comparable -> a) -> RangeMatch comparable a -> RangeMatch comparable a
below bound f cont =
    case cont of
        Next rangeType subject ->
            case rangeType of
                Inclusive ->
                    if subject <= bound then
                        Done (f subject)

                    else
                        cont

                Exclusive ->
                    if subject < bound then
                        Done (f subject)

                    else
                        cont

        Done a ->
            Done a


above : (comparable -> a) -> RangeMatch comparable a -> a
above f cont =
    case cont of
        Next _ subject ->
            Done (f subject)

        Done a ->
            Done a