CODEWITHSUNDEEP
functional-programmingpattern-matchingocaml
HegoDamask
HegoDamask

Reputation: 13

(OCaml: non-exhaustive pattern matching

I have an OCaml function that returns the value of a specified cell in a table. The function works correctly but I still get a warning stating:

Warning 8: this pattern-matching is not exhaustive. Here is an example of a value that is not matched: ([],_)

even though I have in fact accounted for that value in my implementation here:

let cell_value([i;j],table) = match ([i;j],table) with
      ([],_)      -> []
    | (_,[])      -> []
    | (_::_,_::_) -> List.nth (List.nth table (j-1)) (i-1);;

Like I said, the function returns the correct value, I'm just trying to get rid of the error. I'm still fairly new to OCaml, so any help would be greatly appreciated!

Upvotes: 0

Views: 513

Answers (1)

Yann Régis-Gianas
Yann Régis-Gianas

Reputation: 111

Contrary to what you might believe, the problem is not coming from the pattern-matching introduced by the match keyword but by another one you may not be aware of. Indeed, there is a pattern matching in the following fragment of your definition:

let cell_value ([i;j], table) = ...

because ([i; j], table) actually contains the pattern [i; j]. For this reason, this definition assumes that the first component of the pair given as argument to cell_value is a list containing exactly two elements. In other words, this definition is equivalent to:

let cell_value = function ([i; j], table) -> ...

or also to:

let cell_value x = match x with ([i; j], table) -> ...

Now, you probably understand the warning issued by the compiler: if you provide a value of the form ([], table) to your function, it will fail. The compiler is right:

# cell_value ([], []);;
Exception: Match_failure ("//toplevel//", 1, 14).

Upvotes: 7

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