Optional and Lists

-- OptionalAndLists.daml
module OptionalAndLists where

import DA.Optional
import DA.List


-- ── Optional ────────────────────────────────────────────

-- Constructors: Some x  or  None
maybeAge : Optional Int
maybeAge = Some 30

noAge : Optional Int
noAge = None

-- Pattern matching is the safe way to unwrap:
describeAge : Optional Int -> Text
describeAge (Some age) = "Age is " <> show age
describeAge None       = "Age unknown"

-- fromOptional provides a default:
age : Int
age = fromOptional 0 maybeAge   -- 30

age2 : Int
age2 = fromOptional 0 noAge     -- 0

-- optionalToList converts to a zero-or-one-element list:
asList : [Int]
asList = optionalToList maybeAge   -- [30]


-- ── Lists ────────────────────────────────────────────────

-- List literals use square brackets.
names : [Text]
names = ["Alice", "Bob", "Charlie"]

-- Prepend with cons operator `::`
withDave : [Text]
withDave = "Dave" :: names

-- Common operations:
firstItem : Optional Text
firstItem = listToOptional names   -- Some "Alice"

count : Int
count = length names   -- 3

reversed : [Text]
reversed = reverse names

-- zip pairs two lists element-by-element:
numbers : [Int]
numbers = [1, 2, 3]

paired : [(Text, Int)]
paired = zip names numbers
-- [("Alice",1), ("Bob",2), ("Charlie",3)]

-- Comprehension-style with mapA is available in Update context.
-- For pure lists, use map + filter:
shortNames : [Text]
shortNames = filter (\n -> length n <= 3) names

upperNames : [Text]
upperNames = map (\n -> n) names   -- identity; replace with DA.Text.toUpper in practice


-- ── Working with Optional in Update ─────────────────────
-- lookupByKey returns Optional (ContractId T).
-- The safe pattern is:
--
--   result <- lookupByKey @MyTemplate myKey
--   case result of
--     None    -> ... handle missing ...
--     Some cid -> exercise cid MyChoice with ...

Key points

Optional

• Optional a has two constructors: Some a and None. There is no null and no runtime null-pointer error.
• Always pattern match or use fromOptional/optional to extract a value: never assume Some.
• DA.Optional exports: fromOptional, isSome, isNone, mapOptional, catOptionals, optionalToList, listToOptional.

Lists

• Lists in DAML are singly-linked and immutable. :: prepends; ++ concatenates two lists.
• DA.List exports: sortBy, groupBy, dedup, dedupOn, head, last, tail, take, drop, find, partition.
• length, reverse, zip, unzip, concat, concatMap are available without any import.