diff --git a/main.rkt b/main.rkt
index 692ebfb..363766e 100644
--- a/main.rkt
+++ b/main.rkt
@@ -4,6 +4,9 @@
 ; 2018-11-21 - Made by Claudio Maggioni - Tommaso Rodolfo Masera
 
 ; easybf
+(require 2htdp/batch-io)
+(require 2htdp/image)
+(require 2htdp/universe)
 
 ; A Byte is an Int between 0 and 255
 ; Interpretation: a byte in decimal notation.
@@ -45,18 +48,87 @@
        (world-program w)
        (world-ip w)))
 
-; read-file: FileName -> String
-; Given a filename, parses it and returns the content as a string.
-(define (read-file fl)
-  ...)
+; string->program: String -> Program
+; Given a string, returns a bf program without any invalid character
+(define (string->program s)
+  (local [; valid-char: 1String -> Boolean
+          ; Given a valid-char, returns #t if the character is a valid bf
+          ; instruction.
+          (define (valid-char? s)
+            (member? s '(">" "<" "+" "-" "," "." "[" "]")))]
+         (implode (filter valid-char? (explode s)))))
+
+; Tests for string->program
+(check-expect (string->program "hello") "")
+(check-expect (string->program "+world50-[]") "+-[]")
+(check-expect (string->program "") "")
+
+; add1-byte: Byte -> Byte
+; Given a byte, returns the byte+1 simulating overflows
+(define (add1-byte b)
+  (modulo (add1 b) 256))
+
+; Tests for add1-byte
+(check-expect (add1-byte 255) 0)
+(check-expect (add1-byte 254) 255)
+
+; sub1-byte: Byte -> Byte
+; Given a byte, returns the byte-1 simulating underflows
+(define (sub1-byte b)
+  (cond [(zero? b) 255]
+        [else (sub1 b)]))
+
+; Tests for sub1-byte
+(check-expect (sub1-byte 0) 255)
+(check-expect (sub1-byte 1) 0)
+
+; program->world: Program -> World
+; Given a program, returns the corresponding initial world state.
+(define (program->world p)
+  (make-world (cons 0 '()) 0 "" p 0))
+
+; Tests for program->world
+(check-expect (program->world "[->+<]") (make-world (list 0) 0 "" "[->+<]" 0))
+
+; tape-help: Tape DP (Byte -> Byte) -> Tape
+; Given a tape and a data pointer, returns the same tape with the data in the
+; location of the data pointer altered by the function `alter`.
+(define (tape-help tape dp alter)
+  (cond [(zero? dp) (cons (alter (first tape)) (rest tape))]
+        [else (cons (first tape) (tape-help (rest tape) (sub1 dp)))]))
+
+; Tests for tape-help
+(check-expect (tape-help (list 0) 0 add1-byte) (list 1))
+(check-expect (tape-help (list 0 1 2 3) 2 sub1-byte) (list 0 1 1 3))
+
+; exec-add1: World -> World
+; Given a world, returns a new world with the + instruction executed
+(define (exec-add1 w)
+  (make-world (tape-help (world-tape w) (world-dp w) add1-byte)
+              (world-dp w)
+              (world-output w)
+              (world-program w)
+              (add1 (world-ip w))))
+
+; exec-sub1: World -> World
+; Given a world, returns a new world with the + instruction executed
+(define (exec-sub1 w)
+  (make-world (tape-help (world-tape w) (world-dp w) sub1-byte)
+              (world-dp w)
+              (world-output w)
+              (world-program w)
+              (add1 (world-ip w))))
+
+; main: Filename -> World
+; Given a Filename, return the last world state of execution of the
+; corresponding bf program in the file
+(define (main file)
+  (program->world (string->program (read-file file))))
 
 ; TODO:
-; - read-file: FileName -> String
-; - string->program: String -> Program (filter of valid instructions)
-; - program->world: Program -> World (compute initial world state)
 ; - big-bang:
 ; - - - - - - inital state (make-world (list 0) 0 "" <program> 0)
-; - - - - - - on-tick fetch, decode, execute 
+; - - - - - - on-tick fetch, decode, execute
 ; - - - - - - on-key for ","
 ; - - - - - - (on-mouse for stepper)
-; - - - - - - (draw-scene for output and tape)
\ No newline at end of file
+; - - - - - - (draw-scene for output and tape)