This efficiency is why built-in functions to map, filter, etc. vectors to vectors aren't provided; better to use (into []) on a chain of seq-outputting transformations of a vector than to chain these hypothetical vector-native functions.
OTOH, it occurs to me that efficient chaining can still be achieved in library form, by using monads of some sort to transform it into (into [] (op1 (op2 (op3 ... v)))) under the hood. We have something like (into [] (filter x v)), (into [] (map y v)), etc. that we'd like composed into (into [] (filter x (map y (filter z ... v)))), rather than stacking repeated (into [])s, and that pattern *looks* an awful lot like it should be expressible as a monadic transformation of some kind. I don't have enough monad-fu to write out an actual implementation, though. :) That also leads to the weird thought that there might be some way to use monads to create reducers-like functionality for nearly any underlying data structure, with only the need to parametrize a few things for different underlying storage. On Sat, Nov 16, 2013 at 7:11 PM, Andy Fingerhut <andy.finger...@gmail.com>wrote: > I don't know if it is idiomatic, but it certainly looks like a good way to > achieve the desired effect. > > If you chained together calls to several functions like your example > only-evens, it would not be lazy, and it would build up a separate instance > of collections of the original type at each step of the way. Likely it > would be more efficient to delay the conversion back to the original > collection type until after the last sequence operation you wanted to > perform. > > Andy > > > On Sat, Nov 16, 2013 at 4:01 PM, Alexandru Nedelcu > <a...@bionicspirit.com>wrote: > >> Hi, >> >> I'm trying to understand the design of Clojure's collections and one >> thing that I find odd is the return of functions operating on sequences. >> Like for instance a call such as this will return a lazy-seq and not a >> vector: >> >> (drop 2 [1 2 3 4]) >> >> The reason why I find it odd is that data-structures have different >> characteristics and one may want to use a vector because it supports >> efficient indexing and appending to the end. >> >> Of course, dropping 2 elements like above from a vector is probably >> going to have O(n) complexity and thus returning something lazy is more >> appropriate. And while there are some operations, like "conj", "pop" and >> "peek" that preserve the type, functions such as map and filter also >> return lazy-seq. And I worry that the property of the collection you >> start with is lost, given that this returns a "cons": >> >> (conj (filter even? [1 2 3 4 5]) 6) >> >> So lets say that I want to write a generic function that preserves the >> type of that collection. Is something like this idiomatic? >> >> (defn only-evens [coll] >> (into (empty coll) (filter even? coll))) >> >> Thanks, >> >> -- >> Alexandru Nedelcu >> www.bionicspirit.com >> >> PGP Public Key: >> https://bionicspirit.com/key.aexpk >> >> > -- > -- > 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 > Note that posts from new members are moderated - please be patient with > your first post. > 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 > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > -- -- 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 Note that posts from new members are moderated - please be patient with your first post. 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 --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
