!import "prelude" !Local

-- Custom contract combinators built directly on matchResult.
myAndC = (c1 c2 value rest :
  matchResult
    (msg _ : contractErr msg rest)
    (v _ : c2 v rest)
    (c1 value rest))

-- Plain predicates lifted into contracts with a diagnostic message.
natural? = guardC "natural" (n : gte? n 0)
nonZero? = guardC "non-zero" (n : not? (isZero? n))

-- Phantom annotations let point-free definitions carry their own contracts.
-- The base library now uses the same syntax, so head/tail/div etc. are
-- guarded by default.
myHead @(nonEmptyListOf anyC) =@anyC head
myTail @(listOf anyC) =@(listOf anyC) tail
myDiv @natural? @(myAndC natural? nonZero?) =@natural? div

-- The `check` helper applies a contract to any value and returns the
-- checked value (or the diagnostic message on failure).
checkedSuccessor = check natural? (add 1 2)

main = pair checkedSuccessor (myDiv 10 2)