Bend/example.hvm

// Write definitions like this
(Def1) = ((λa a) (λb b))

// You can call a definition by just referencing its name
// It will be substituted in place of the reference
(Def2) = ((λa a) Def1)

// Definitions and variables can have names in upper and lower case and contain numbers
// Names defined in a more inner position shadow names in an outer position
(def3) = ((λDef1 Def1) (λx λx x))

// The language is affine, but if you use a variable more than once the compiler inserts duplications for you
// Of course you can always do them manually
(def4) = λz dup z1 z2 = z; (z1 ((λx (x x x x x)) z2))

// You can use machine numbers and some native numeric operations
// Numeric operations can only reduce numbers, doing (+ (λx x) 1) will not do anything
(nums) = λx1 λx2 (* (+ x1 1) (/ (- x2 2) 1))

// You can use numbers on the native match expression
// The `+` arm binds the `scrutinee`-1 variable to the the value of the number -1
(Num.pred) = λn 
  match n { 
    0: 0
    +: n-1
  }

// Write new data types like this
data Option = (Some val) | None
data Bool = True | False

// You can have pattern matching on definitions
// Use `*` to ignore a pattern
(Option.unwrap_or (Some val) *) = val
(Option.unwrap_or None      or) = or

(Bool.or True  *) = True
(Bool.or * True)  = True
(Bool.or * *)     = False

// Or using a match expression
(Bool.not) = λbool
  match bool {
    True:  False
    False: True
  }

// Data types can store values
data Boxed = (Box val)

// Types with only one constructor can be destructured using `let` or a single matching definition
(Box.map (Box val) f) = (Box (f val))

(Box.unbox) = λbox
  let (Box val) = box
  val

// Use tuples to store two values together without needing to create a new data type
(Tuple.new fst snd) =
  let pair = (fst, snd)
  pair

// Then you can destructure it inside the definition or using `let`
(Tuple.fst (fst, snd)) = fst

(Tuple.snd) = λpair
  let (fst, snd) = pair
  snd

// All files must have a main definition to be run.
// You can execute a program in HVM with "cargo run -- --run <path to file>"
// Other options are "--check" (the default mode) to just see if the file is well formed
// and "--compile" to output hvm-core code.
(main) = 
  let tup = (Tuple.new None (Num.pred 5))

  let fst = (Tuple.fst tup)
  let snd = (Tuple.snd tup)

  let box =     (Box fst)
  let map =     (Box.map box Option.unwrap_or)
  let unboxed = ((Box.unbox map) snd)

  (nums 3 unboxed)