CODEWITHSUNDEEP
pattern-matchingocaml
power_output
power_output

Reputation: 421

Compare two integer lists in OCaml

I want to compare two integer lists. I started with pattern matching but I had a problem with nesting matches so tried another way.

I'm getting warnings that pattern-matching is not exhaustive, it says that list can be empty. That's strange since I check that at the beginning.

let rec cmp3 l1 l2 = 
    if l1 = [] && l2 = [] then 0 
    else if l1 = [] then -1
    else if l2 = [] then 1 else 
    let (h::t) = l1 and (hh::tt) = l2 in
    if h > hh then 1 
    else if hh > h then -1 
    else cmp3 t tt;;
              Characters 125-131:
      let (h::t) = l1 and (hh::tt) = l2 in
          ^^^^^^
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
[]
Characters 141-149:
      let (h::t) = l1 and (hh::tt) = l2 in
                          ^^^^^^^^
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
[]
val cmp3 : 'a list -> 'a list -> int = <fun>

Upvotes: 2

Views: 9540

Answers (1)

Pierre G.
Pierre G.

Reputation: 4431

The compiler cannot assume that both list have the same length - this is why it issues a warning. If you are confident that your list have always the same length, then up to you to discard this warning - but that is not a safe way to write programs.

Also you have many ifs, it is better to use match, it is more readable. As an example :

let rec cmp l ll = 
match (l,ll) with
| [], [] -> 0
| [],_ -> -1
| _,[] -> 1
| (h::t), (hh::tt) -> if h > hh then 1
                      else if h < hh then -1 
                      else cmp t tt;;

Upvotes: 3

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