Re: [Haskell-beginners] Maybe and Just

For what its worth I wrote this a while back, a hopeless attempt to do something "monad like" in PHP but reading it again..... well, it's no wonder I like Haskell more than I like having to use PHP! http://objitsu.blogspot.co.uk/2013/01/php-and-maybe-miserable-monads.html On 26 March 2015 at 10:26, Michael Alan Dorman wrote:

Shishir Srivastava writes:

...

After reading and re-reading the haskell tutorials I don't happen to see a very convincing or appealing reason for having these data types.

To be clear: Maybe is the data *type*. Just and Nothing are its data *constructors*.

...

Can anyone please explain where Maybe and Just provide the sort of functionality that cannot be achieved in other languages which don't have these kind.

The functionality can be achieved in other languages, certainly. The question is whether the clarity and safety is also achieved.

When I see (as a totally contrived example):

fopen :: Maybe FileHandle

I know that that function may not be able to return a FileHandle value all the time. The compiler will, in fact, nag me if I do not write the code that calls it in such a way that it acknowledges that possibility.

When I see:

FILE * fopen ( const char * filename, const char * mode );

It is not immediately clear whether that can fail. Sure, we can make that inference, based on what we know about filesystems, etc., but the compiler is never going to complain if I ignore the possibility.

In my experience, programmers in many languages end up resorting to convention to try and work around these sorts of ambiguities. Large projects have strategies for naming functions that try to pass along information out of band, or languages have a pervasive culture of "lint" tools that try to use heuristics to make up for what the type system doesn't make simple.

That said, I know that doing Maybe sorts of things in languages that don't have, say, pattern matching, or the idea of a "failure monad", gets to be a drag very quickly---manually unwrapping things is at best awkward, having to re-wrap them just to unwrap them again in a sequence of computations quickly leads one to believe "it's just not worth it"---or you resort to exception handling, which has its own challenges to do well.

Mike. _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners