I'm working on a short project to convert small programs from python to java and visa versa. I created the following code, and tested it in utop.
let c =
let x = "for (int i = 0; i<10; i++)"
and y = "for i in range(0,10):"
in function
| x -> y
| y -> x
| _ -> "Oh no!!";;
For some reason, x & y are both considered unbound, yet at the same time any parameter seems to match to x.
What order does everything need to be written in to make this work?
Simply to follow up with your answer.
In pattern-matching, matching to a variable doesn't necessarily seem to match to its value.
This is the very reason why it's called pattern-matching and not value-matching.
As the name implies, pattern-matching is used to match things against patterns, not values. In the code you show in the question, you are not actually comparing anything to x or y, you are defining patterns named x and y that can match anything. See the example below:
match 2 with
| x -> "Hey, I matched!"
| _ -> "Oh, I didn't match.";;
- : string = "Hey, I matched!"
Note that this works even if
xwas previously defined. In the match case, thexfrom the pattern is actually shadowing the other one.let x = 42 in match 1337 with | x -> Printf.printf "Matched %d\n!" x | _ -> ();; Matched 1337! - : unit = ()
On the other hand, the pattern i when i = x is actually matching against the value of the outer variable x, which is why the code in your self-answer works. But this is not what patterns are for anyway.
What you're actually trying to do is not a pattern-matching, it is a simple conditional statement.
let c argument =
let x = "for (int i = 0; i<10; i++)" in
let y = "for i in range(0,10):" in
if argument = x then y
else if argument = y then x
else "Oh no!";;
val c : string -> string = <fun>
And here it is in action:
c "for (int i = 0; i<10; i++)";;
- : string = "for i in range(0,10):"
c "for i in range(0,10):";;
- : string = "for (int i = 0; i<10; i++)"
c "whatever";;
- : string = "Oh no!"
Also, don't use and unless you're defining mutually recursive values.