PM>type Exp = Const of float | Plus of (Exp * Exp) | ...
PM>

type Operator = Plus|Minus|Mult|Div|...| Custom of string
type Expr = Literal of float | Identifier of string | Unary of (Operator*Expr) | Binary of (Operator*Expr*Expr)

unop : Operator -> (float -> float)
binop : Operator -> (float -> float -> float)

unop o = match o with
    | Plus -> (+.) 0
    | Minus -> (-.) 0
    | _ -> error "Unexpected unary operator"
binop o = match o with
    | Plus -> (+.)
    | Minus -> (-.)
    | Mult -> (*.)
    | Div -> (/.)
    | _ -> error "Unexpected binary operator"

eval expr = match expr with
    | Literal c -> c
    | Identifier n -> lookupValueSomewhereElse n
    | Unary (o,x) -> unop o (eval x)
    | Binary (o,x,y) -> binop o (eval x) (eval y)

ast2fun : Expr -> (Set of string) -> (Set of string)*(Map of string*float -> float)
    (* на входе выражение и множество уже найденных переменных *)
    (* на выходе - множество входящих в него переменных *)
    (* и функция, принимающая таблицу значений *)

(* типы Set и Map - вымышленные. смотрите их аналоги в документации *)

ast2funS e s = match e with
    | Literal c -> (s, const c)
    | Identifier n ->
        let sn = append s n in
        let f m = lookup m n in
        (sn, fn)
    | Unary (o,ex) ->
        let fo = unop o in
        let (sx,fx) = ast2funS ex s in
        let f m = fo (fx m) in
        (sx, f)
    | Binary (o,ex,ey) ->
        let fo = binop o in
        let (sx,fx) = ast2funS ex s in
        let (sy,fy) = ast2funS ey sx in
        let f m = fo (fx m) (fy m) in
        (sy, f)

ast2fun e =
    let (_,f) = ast2funS e emptySet
    in f