On Thu, Jan 9, 2014 at 10:26 AM, Vlatko Basic <vlatko.basic@gmail.com> wrote:

I put String because I'm currently thinking about error handling, and Left String is the usual way of reporting failure, and I see Maybe as a type for reporting errors, failures and similar.

Actually, the fact that all you can convey is "something failed" makes Maybe not a good error reporting type. And this is fine; there is still the "no value" niche (Perl's undefined, SQL's NULL, etc.) --- and the evidence from C's NULL that an *out of band* representation is often a very good idea (and from IEEE754's NaN that multiple out of band values is often a very bad idea).

Also, a point that seems to often be missed in considering what is "more important": ultimately, what is *most* important is letting the programmer do what they need to do with a minimum of fuss or extra work. Predefining very useful stuff like Maybe which could be reinvented on the fly based on Either is about minimizing even the trivial extra work. Pedagogy is only rarely a useful design goal.

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

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