RE: [Caml-list] ocaml 4.03 and warning 52 : argument of this constructor should not be matched against a constant pattern

[David Allsopp <dra-news@metastack.com>]

William wrote:
> Hello,
> 
> with ocaml 4.03, I came with many warnings such as this one :
> 
> let lst = [] in
> try
>     let fst = List.hd lst in
>     Printf.printf "fst:%s\n" fst
> with
> | Failure "hd" -> Printf.printf "empty list\n";;
> 
> Characters 123-127:
> | Failure "hd" -> Printf.printf "empty list\n";;
>             ^^^^
> Warning 52: the argument of this constructor should not be matched against
> a constant pattern; the actual value of the argument could change in the
> future.
> 
> 
> 
> It occurs typically for Failure "hd", Failure "tl", Failure
> "float_of_string", Failure "int_of_string", Failure "lexing: empty token",
> Invalid_argument "Filename.chop_extension", ...
> 
> How are we supposed to handle this nicely with the standard library ? Is
> there a better approach to catch those exceptions ?

In most (hopefully all) of those instances, what you're matching is Failure - i.e. the function in question only raises it in one instance, so I think the idea now is that you should match against [Failure _] rather than including the string in the match. failwith is used in too many places in the Standard Library, but that of course can't now be changed (cf. ExtList.List.Empty_list and others)

That said, Failure "lexing: empty token" is an odd one, because I certainly have lexers which have to emulate that...


David