datatype expression =  Constant of int |
        Variable of string |
        Operator of string * expression |
        Pair of expression list |
        List of expression list

datatype pattern = ConstantP of int
    | VariableP of string
    | OperatorP of string * pattern
    | PairP of pattern list
    | ListP of pattern list
    | Wildcard

(*match (fn : expression * pattern -> (string * expression) list option) -
accepts  expression and a pattern. In case of matching returns SOME list of matching in case of not matching it returns NONE.

We can define matching on pair of (expressin * pattern). Pattern and expression can match or not. If they match we create list of matchings of tuples of names and values ((string * expression) list).. the order of matching list doesnt matter.

The rules for matching:

Wildcard matches all elements and returns empty list of matchings
VariableP s matches with any value of v and returns a list with one binding (s,v)

ConstantP 42 matches only with Constant 42 and returns empty list of bindinds. It works the same for all other numbers.


PairP [a1, a2] matches with Pair [b1,b2] if a1 and b1 matches and a2 and b2 matches.. It returns list of all matches.

ListP ps matches with List xs if all elements matches. Match returns a list of all matches.

*)