(OCaml: non-exhaustive pattern matching

Question

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!

Your signature is weird. It is much more common to use a tuple in this kind of application than a list. The following would make more sense and would be a lot simpler too : let cell_value((i,j),table) = ... — ghilesZ

Yann Régis-Gianas Yann Régis-Gianas · Accepted Answer · 2018-04-08T15:42:30

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).

(OCaml: non-exhaustive pattern matching

1 Answers