Plai 2:everything about Parser

Plai-type's parser, have to say that Lisp is still good at dealing with parentheses expression, and its list of member allow different types, but really, still do not like Lisp.

#lang plai-typed (Define-type arithc  [NUMC (N:number)]  [PLUSC (L:ARITHC) (R:ARITHC)]  [MULTC (L:ARITHC) (R:ARITHC)]) (Define (parse [s:s-expression]): arithc  (Cond    [(S-exp-number s) (NUMC (S-exp->number s))]    [(s-exp-list? s)     (Let ([SL (S-exp->list s)])       (Case (S-exp->symbol (first SL))         [(+) (PLUSC (Parse (second SL)) (Parse (third SL))]         [(*) (MULTC (Parse (second SL)) (Parse (third SL))]         [Else (Error ' Parse ' invalid list input ')])]    [Else (Error ' Parse ' invalid input ')])) (Parse ' (+ (* 1 2) (+ 2 3)))

Because just started to learn OCaml, so also write a parser with OCaml, but I use the state machine to achieve, very ugly, verbose, and feel the code style or imperative, away from the functional or there is a great distance.

Let S = "(+ (* 1 2) (+ 2 3)" Inlet L = string.length s inlet state:int ref = ref 0 Inlet Rec Parse (i:int) = if I >= L then () Else (match S.[i] with |                ' (' Print_char ' (';                    If!state = 0 Then (state: = 1;                Parse (i+1); ) Else If!state = 3 | |                    !state = 5 Then (state: = 1;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); )        |                    ' + ' if!state = 1 then (print_string "PLUSC";                    state: = 2;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); ) |                    ' * ', if!state = 1 then (print_string "MULTC";    state: = 2;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); )        |                "Print_char";                    If!state = 2 Then (state: = 3;                Parse (i+1); ) Else If!state = 4 | |                    !state = 7 Then (state: = 5;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); )        |                ') ', Print_char ';                    If!state = 7 then (state: = 7;                Parse (i+1);                    else if!state = 6 then (state: = 7;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); ) | C-Printf.printf "(NUMC%c)" C;                    If!state = 3 then (state: = 4;                Parse (i+1);                    else if!state = 1 then (state: = 8;                Parse (i+1);                    else if!state = 5 then (state: = 6;                Parse (i+1);                    ) Else (print_string "error";                Parse (i+1); )) in parse 0

　　

Plai 2:everything about Parser