Re: [Caml-list] Fragile pattern matching?!

[Alex Baretta <alex@barettadeit.com>]

Keith Wansbrough wrote:
> Alex Baretta writes:
> 
> 
>>let to_int value = match value with
>>   | Int x -> x
>>   | _ -> raise Some_exception
>>
>>The compiler signals a warning for a fragile pattern matching at the "_" 
>>character.
>>
>>Why in the world should this code signal such a warning?
> 
> 
> The reason is to do with code maintenance.  When in future you add
> another constructor to the value type, the type checker will tell you
> the locations of all matches that have now become incomplete, so that
> you can fix them.  But the match in "to_int" above will never become
> incomplete.  If the type checker passed this silently, then a bug
> could easily be introduced.  Hence the warning.

I understand the need for this kind of warning, but it seems to me that 
I have used the "match x with Something -> do_something x | _ -> 
raise_an_exception" for years. Suddenly, I see the compiler signal 
warnings where I would not expect to seen them.

What is the exact definition of fragile pattern matching?

Alex


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