Patterns and logic

Wipple has a concept of “patterns”, which bind to one of many options for a piece of data. For example, we can define a Maybe type:

Maybe : A => type {
    Some A

use Maybe

And then bind to a Some or None value:

x? : Some 42
Some x : x? -- x : 42

If x doesn’t contain a Some value, the program crashes:

x? : None
Some x : x? -- runtime error

You can use patterns in function parameters too!

unwrap : Some x -> x -- unwrap :: for A -> Maybe A -> A

On its own, this isn’t very useful — why would you want your program to crash? To get around this, we can use Wipple’s fundamental logic operation, when. You give when a bunch of functions, and it will call the one whose pattern matches the input:

Grade : type {

use Grade

when grade {
    A -> "Top of the class"
    B -> "Pretty good"
    C -> "Getting there"
    D or F -> "Need to study"

Thanks to when, Wipple doesn’t have regular booleans! Boolean logic is implemented using the Boolean type:

Boolean : type {

use Boolean

if : bool then else ~> when bool {
    True -> then
    False -> else

show (format "x _ 5" (if (x = 5) "is" "is not"))

You can bind variables inside when like so:

Map : f -> x? -> when x? {
    Some x -> Some (f x)
    None -> None

area : when shape {
    Square s -> s ^ 2
    Rectangle l w -> l * w
    Circle r -> 3.14 * r ^ 2

The cases you provide to when must be exhaustive (ie. they must cover all variants of the type). To ignore some cases, or to handle multiple cases in one branch, you can use _ and or:

when color? {
    Some (Red or Blue) -> "hooray"
    _ -> "oh no"

Alternatively, you can use when?:

when? color? (Some (Red or Blue)) (show "hooray")

If you just want to execute a piece of code when a condition is true, you can use when!:

when! (2 + 2 = 4) (show "woohoo")

The opposite form is unless!:

unless! : template bool body -> when! (not bool) body