Functions and Let

-- FunctionsAndLet.daml
module FunctionsAndLet where

-- ── Basic function ──────────────────────────────────────
add : Int -> Int -> Int
add x y = x + y

-- Partial application: fix one argument.
addFive : Int -> Int
addFive = add 5

result : Int
result = addFive 3   -- 8


-- ── let / in ────────────────────────────────────────────
-- let binds local names; `in` introduces the body expression.
circleArea : Decimal -> Decimal
circleArea r =
  let
    pi = 3.14159265358979
    rSquared = r * r
  in
    pi * rSquared


-- ── Guards ──────────────────────────────────────────────
-- Guards are conditions checked top to bottom.
-- `otherwise` is an alias for True and always matches.
classify : Int -> Text
classify n
  | n < 0     = "negative"
  | n == 0    = "zero"
  | n < 100   = "small positive"
  | otherwise = "large positive"


-- ── Pattern matching ────────────────────────────────────
data Shape = Circle Decimal | Rectangle Decimal Decimal
  deriving (Show)

area : Shape -> Decimal
area (Circle r)        = 3.14159265358979 * r * r
area (Rectangle w h)   = w * h


-- ── Lambdas ─────────────────────────────────────────────
-- Backslash introduces a lambda. Arrow separates params from body.
double : [Int] -> [Int]
double xs = map (\x -> x * 2) xs

-- Equivalent using a named helper:
doubleItem : Int -> Int
doubleItem x = x * 2

double2 : [Int] -> [Int]
double2 xs = map doubleItem xs


-- ── Standard list combinators ────────────────────────────
evens : [Int] -> [Int]
evens xs = filter (\x -> x % 2 == 0) xs

total : [Decimal] -> Decimal
total xs = foldl (+) 0.0 xs

-- map applies a function to every element
-- filter keeps elements satisfying a predicate
-- foldl reduces a list to a single value (left-associative)

Key points

Functions

Functions are curried: add : Int -> Int -> Int is a function that returns a function. add 5 is valid and returns an Int -> Int.
There are no for, while, or do/while loops. Iteration uses map, filter, foldl, or recursion.
Lambdas are first-class values and can be passed to and returned from functions.

Control flow

let bindings inside a do block do not use in — only top-level let/in needs it.
Guards are evaluated top-to-bottom; the first matching guard wins.
show : a -> Text converts any value with a Show instance to its string representation.