CODEWITHSUNDEEP
recursionf#
Javier Lorenzini
Javier Lorenzini

Reputation: 179

F# Recursive function not ending although I think it should

I have a recursive function in my code that looks like this:

let rec func (a,b)
    match (a,b) with
    | condition 1 -> func a+1 b
    | condition 2 -> print "%s" done
    | _ then func a b+1              //<-- It enters here after going into the else ()!!
    if a = b then func 0 0
    else ()

The problem is that when the match section ends, it goes to the if statement and doesn't match it, which is good. So it goes to "else ()" and should quit the recursion but instead it goes back to the |_ -> print func a b+1. Why can this happen?

Upvotes: 0

Views: 128

Answers (3)

Phillip Scott Givens
Phillip Scott Givens

Reputation: 5464

You effectively have two sections of code

a match statement

 match (a,b) with
 | condition 1 -> func a+1 b
 | condition 2 -> print "%s" done
 | _ then func a b+1              //<-- It enters here after going into the else ()!!

and then an if/else

 if a = b then func 0 0
 else ()

Unless "condition 2" is matched, this function will always recurse during the match section and it does not matter that the if/else block does not recurse. I am not sure what you are attempting to achieve, but maybe you want a match/when

 match (a,b) with
 | condition 1 -> func a+1 b
 | condition 2 -> print "%s" done
 | _,_ when a = b -> func 0 0
 | _ -> func a b+1              //<-- It enters here after going into the else ()!!

Now "condition 2" is your stop case. Is "condition 2" an active pattern?

Upvotes: 1

Javier Lorenzini
Javier Lorenzini

Reputation: 179

I got where my problem was. The recursion was called N times, so when the recursion ended within the match statement, it had to go N times into the if statement. I don't know why while debugging it would show the |_ -> else() highlighted, but I already understood the problem and solved it. Thank you guys!

Upvotes: 0

S&#248;ren Debois
S&#248;ren Debois

Reputation: 5688

Presumably, after reaching

| condition 2 -> print "%s" done

then if a=b your program continues by doing 

if a = b then func 0 0

(Or maybe it returns, then do the same call higher up the call chain.) Likely that recursive call then takes you to 

 | _ then func a b+1

Because of lightweight syntax, the if after the match is always executed.

Upvotes: 0

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