CODEWITHSUNDEEP
ocaml
Letane
Letane

Reputation: 1

How to fix pattern matching recursive call for tuples OCAML?

I'm coding a helper function for encodeHelper but cannot seem to get the pattern matching to work in the recursive call. Does anyone know how to fix this error?

Please look at the code and error message in UTOP

let rec encodeHelper (l : 'a list) (curr : ('a * int) list) (currElement : ('a * int) option) =
   match l, currElement with 
   | [], None -> curr
   | [], Some(k) as currElement -> curr @[(fst k, snd k)]
   | hd1::tl, None -> encodeHelper tl curr (k, 1)
   | hd::tl, Some(k) -> 
      let element_ = fst k and
      amount = snd k in
      if element_ = hd
         then encodeHelper tl curr (element_, amount+1)
      else
         encodeHelper tl (curr @ [(element_, amount)]) (hd, 1)

The error message is that (k,1) in the recursive call on the 5th line is This expression has type 'b * 'c but an expression was expected of type ('a * int) option

Let me know if you have tips / ways to fix this!

Upvotes: 0

Views: 66

Answers (1)

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66818

The compiler is telling you that this expression is wrong:

encodeHelper tl curr (k, 1)

Indeed, the third argument of encodeHelper is specified to be of type ('a * int) option and the expression (k, 1) can't possiblly be of this type. An option type is None or Some x.

Possibly it will work better if you replace (k, 1) with (Some (k, 1)). However I don't see a definition of k in this branch of the match.

Upvotes: 2

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