Re: FiniteMap: modifyFM

Hi all, On 19 Jun 2002, Ketil Z. Malde wrote:

Alastair Reid writes:

OTOH, I find I define classes very sparingly in my own programming. Comong from an OO world I started out with a desire to create classes for just about anything, but in practice, it rarely seems necessary. So while it still can be useful or desirable, it just doesn't get done.

There are always (as I see it) three options: - Use a class -- i.e., Set s a => empty :: s a - Use a "qualified" name -- i.e., S.empty :: Set a - Use a name with a qualifier tacked on manually -- i.e., emptySet :: Set a Going from the bottom up, we have the least complex up to the most complex. The qualified name option has as a problem (pointed out recently, I don't remember whom by) the issue of collection modules. I think the general policy I've used is: if there is already a class into which it fits, put it in a class; otherwise, put use the third option. This is largely due to the fact that the Prelude was so well designed. Usually, whatever my data type, the operations I wish to perform over it fall into some function from the prelude, most usually: map, fold, filter (though less often filter than the other two), mapAccum, mapM, foldM, lookup, find, and maybe a few otheres (For instance, I wrote all of these functions to work on top of the HsSyn library of Haskell syntax to easy some work I was doing.) The problem is that the prelude is so well put together than I rarely need more functions than it provides for my datatypes (at least, more *general* functions; obviously each data type has data-specific functions, but I'm glossing over that now). The problem this leaves me with is that it's taken all the good names. So I end up calling the functions on my trees: tmap, tfold, tlookup, etc... (Though occasionally I use fmap in Functor instead of tmap.) This extends beyond collections; I recently needed a datatype (silly as it may sound), which is |R U {+oo, -oo}, with addition defined as: -oo + _ = -oo oo + _ = oo r + r = r + r (obviously associative). That is, -oo always wins out, even over +oo, but otherwise it's just "normal" addition. The problem is, I either had to use some strange symbol like $+ for addition, or make it an instance of Num, without any of the other operations (I opted for the latter). I've been considering both (a) using Ashley's prelude, which seems to adopt a one-op-per-class mentality or (b) learning more about Clean. I know there was some talk a while back about having a "beginners" prelude (similar to what we have now) and an "advanced" prelude, supposedly similar to Ashley's. I would definately be in favor of this. Whether we call it class Add or class Group is largely irrelevant to me (I tend to find class names to be fairly superfluous anyway). I think the only classes I ever write (or derive) instances of are: Eq, Ord, Show, Read (rarely), Num, Binary, Fractional (rarely), Hash, DeepSeq, Monad (rarely), Functor (rarely) Okay, well I guess that's almost all of them. But according to a quick grep, the only onces I write frequently are: Eq, Ord, Show, Num Moreover, the only *classes* I've ever defined (other than for toy little tests with classes) are: Hashable, Container Though the functions I frequently want to override (listed above) fall into none of these. So...yeah...I've kind of lost my train of thought, but I think that the problem I've frequently had is that I want to use the names the prelude uses, but can't. :). That's all. - Hal