2009/3/3 Mark Volkmann <r.mark.volkm...@gmail.com> > > On Tue, Mar 3, 2009 at 9:12 AM, Laurent PETIT <laurent.pe...@gmail.com> > wrote: > > Hello Mark, > > > > Just one point : > > > > 2009/3/3 Mark Volkmann <r.mark.volkm...@gmail.com> > >> > >> Does this seem like a good way to choose between doall, dorun, doseq > >> and for to evaluate all the items in sequences? > >> > >> Ask these questions: > >> > >> Do you already have the lazy sequence in a variable or do you still > >> need to build it? > >> If you already have it, use dorun or doall. Otherwise use doseq or for. > >> While code inside a dorun or doall could build the sequence, using > >> doseq and for are considered more idiomatic/readable. > >> Also, they provide list comprehension features such as processing more > >> than one sequence and filtering with :when/:while. > >> > >> For example, instead of using the following to get a new sequence > >> where all the items are multiplied by two: > >> (doall (map #(* % 2) my-coll)) > >> use this: > >> (for [item my-coll] (* item 2)) > > > > I don't think it is a good example, since it conveys the idea that it > could > > be interesting to use doall or for for mapping a coll to multiply its > items. > > This example, in which there is no side effect at all in the inner loop, > is > > really typical of the use of map ! And forcing the sequence there gives > you > > nothing (?) > > > > Maybe a more interesting example could be something that touches global > > vars, or does IO, ... ? > > Very good point! With what I'm doing, simply multiplying items by two, > you'd want the result to be an unevaluated, lazy sequence which is why > map would be preferred. > > But supposing I was doing something like you suggested where I really > do want to force the evaluation of all the items in the sequence, do > my rules of thumb make sense?
I haven't thought really hard about it, what it written seems correct to me. But I think the first point is a little bit dangerous without a side note : "if you already have the seq, then call 'doall or 'dorun on it". Indeed, if you're dealing with a seq that is passed around, you may end up calling dorun or doall on the seq in several places in the code, which may lead to problems if you have side effects in them ! So to prevent this risk : either the call to 'doall or 'dorun is factorized, and it will certainly be factorized near the place where the seq is generated (thus almost falling back to your second case "you don't have generated the seq yet"), either you haven't side effects at all, and you fall back to an idiomatic use of 'map or another lazy-seq generating function, without the need to call 'doall or 'dorun on them. HTH, -- Laurent > > > -- > R. Mark Volkmann > Object Computing, Inc. > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
