On Sun, Jan 16, 2011 at 6:22 AM, Jürgen Hötzel juer...@hoetzel.info wrote:
Hi,
I came across this issue while implementing a lazy, efficient flatten that
also uses the whole sequence abstraction (flatten java arrays).
The problem with (seq x) is, that it will throw an Exception if called on
something, that cannot be coerced to sequence, so I just used sequencial?
like the current implementation of flatten:
(defn my-flatten [coll]
(mapcat (fn [p] (if (sequential? (first p)) (mapcat my-flatten p) p))
(partition-by sequential? coll)))
But this will obviously fail to flatten anything that doesn't
implement clojure.lang.Sequential (like java.arrays).
Unfortunately, right now that's a big gap in the clojure.core
functionality. It's quick to fix, though the fix is a bit of a hack as
it uses the exception-throwing behavior of (seq x). On the plus side
this guarantees that it meets its contract.
(defn seqable? [x]
(try (seq x) (catch Exception _)))
(def m-seq seqable?)
(defn seqable-ns? [x]
(if-not (instance? java.lang.String x) (seqable? x)))
(def m-seq-ns seqable-ns?)
Anything seqable is returned unchanged by seqable? and m-seq, which
return nil rather than throw if the thing is not seqable. The two
names for the same function are just to improve code readability:
seqable? for use as a predicate and m-seq for uses like
(if-let [x (m-seq foo)] (do-something-with x))
The -ns versions differ only in that they pretend Strings aren't
seqable, for those fairly common cases where you want to treat strings
as atomic rather than as character sequences; for instance if you're
probing a data structure made of nested colls and wish to regard whole
strings as leaf nodes in the traversal instead of their individual
characters.
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