-- The parametrized module NO-DUP-LIST(ELEMENTS :: TRIV) defines the signature of simple Haskell like list structure.
-- The removal of duplicates is handled by the equational properties listed after the signature in brackets {}
-- The binary operation _,_ is associative, commutative, and idempotent.
-- This list structure does not permit duplicates, they are removed during evaluation (called reduction in CafeOBJ)
-- Actual code is contained in module called  NO-DUP-LIST.
-- The tests are performed after opening instantiated NO-DUP-LIST with various concrete types.
-- For further details see: http://www.ldl.jaist.ac.jp/cafeobj/
mod! NO-DUP-LIST(ELEMENTS :: TRIV)  {
    [ List < Elem < Elt]  -- Sorts in Ordered Sorted Algebra
    op [] : -> List { prec: 0 }  -- Empty List
    op _,_ : Elt Elt -> Elt { comm assoc idem prec: 80 l-assoc }
    op [_] : Elt -> List  { prec: 0 }
}

-- Test on lists of INT, CHARACTER, and STRING
open NO-DUP-LIST(INT)
reduce [ 1 , 2 , 1 , 1 ] .     -- Gives [ 1 , 2 ]
open NO-DUP-LIST(CHARACTER)
reduce [ 'a' , 'b' , 'a' , 'a' ] .  -- Gives [ 'a' , 'b' ]
open NO-DUP-LIST(STRING)
reduce [ "abc" , "def" , "abc" ] . -- Gives [ "def" , "abc" ]