unison/unison-src/basics.u

-- Unison is a statically typed functional language

increment : Nat -> Nat -- signature is optional
increment n = n + 1

-- Lines starting with `>` are evaluated and printed on every file save.
> increment 99

replicate : Nat -> a -> [a]
replicate n a = toSequence (take n (constant a))

-- this is nice for quick testing!

> replicate 3 "bye"

-- can ask Unison for the type of any expression just by adding `?` to the end of it

-- > (replicate 4)?

-- here's a more interesting example, mergesort -

-- First we define the merge function, it merges two sorted lists
merge : (a -> a -> Boolean) -> [a] -> [a] -> [a]
merge lte a b =
  use Sequence ++
  use Optional None Some
  go acc a b = case at 0 a of
    None -> acc ++ b
    Some hd1 -> case at 0 b of
      None -> acc ++ a
      Some hd2 ->
        if hd1 `lte` hd2 then go (acc `snoc` hd1) (drop 1 a) b
        else go (acc `snoc` hd2) a (drop 1 b)
  go [] a b

-- let's make sure it works
> merge (<) [1,3,4,99,504,799] [0,19,22,23]

-- looks good, now let's write mergesort

sort : (a -> a -> Boolean) -> [a] -> [a]
sort lte a =
  if Sequence.size a < 2 then a
  else
    l = sort lte (take (size a / 2) a)
    r = sort lte (drop (size a / 2) a)
    merge lte l r

-- let's make sure it works

> sort (<) [3,2,1,1,2,3,9182,1,2,34,1,80]

> sort (<) ["Dave", "Carol", "Eve", "Alice", "Bob", "Francis", "Hal", "Illy", "Joanna", "Greg", "Karen"]

-- SHIP IT!! 🚢

-- If you make a mistake, we try to have nice friendly error messages, not:
-- 🤖 ERROR DETECTED ⚡️ BEEP BOOP ⚡️ PLS RESUBMIT PROGRAM TO MAINFRAME

-- a few examples of failing programs -

--err1 =
--  a = "3"
--  sort (<) [1,2,a]

-- err1a = sort (<) "not a list"

--err2 : x -> y -> x
--err2 thing1 thing2 =
--  if true then thing1
--  else thing2