On Feb 18, 12:20 pm, Frantisek Sodomka <fsodo...@gmail.com> wrote:
> How should I say it... It just didn't look "symmetrical" to me.
>
> So, basically, there is a difference between functions returning
> sequences - depending on if they are lazy or eager. Hmm...
>
> user=> (reverse [])
> nil
> user=> (if (reverse []) true false)
> false
> user=> (if (seq (reverse [])) true false)
> false
>
> user=> (lazy-seq nil)
> ()
> user=> (seq (lazy-seq nil))
> nil
> user=> (if (lazy-seq nil) true false)
> true
> user=> (if (seq (lazy-seq nil)) true false)
> false
>
> As long as I remember which function is lazy and which one is eager, I
> should be fine then.
>
> Just wanted to really understand it.
>
It shouldn't be that subtle. Sequence functions shouldn't return nil
unless they are variants of seq/next. I've fixed reverse and sort to
return () when passed empty colls - SVN 1294.
Rich.
>
> On Feb 18, 5:58 pm, Chouser <chou...@gmail.com> wrote:
>
> > On Wed, Feb 18, 2009 at 11:46 AM, Frantisek Sodomka <fsodo...@gmail.com>
> > wrote:
>
> > > What about 'conj'? Documentation says:
> > > (conj nil item) returns (item).
>
> > > Currently:
> > > user=> (conj nil 1)
> > > (1)
> > > user=> (conj () 1)
> > > (1)
>
> > Is there something wrong with that? It looks right and like it
> > matches the docs to me.
>
> > > Idiom "conj nil" is used in 'reverse': (reduce conj nil coll)
> > > Currently:
> > > user=> (reverse [1 2])
> > > (2 1)
> > > user=> (reverse [1])
> > > (1)
> > > user=> (reverse [])
> > > nil
>
> > > It looks that now all sequence functions return () instead of nil. Is
> > > 'reverse' correct?
>
> > Things that return lazy seqs now return an empty lazy seq, which
> > prints as (), instead of nil. However, 'reverse' is not lazy and
> > normally returns a PersistentList. I don't know that it'd be more
> > correct to return an empty PersistentList than to return nil as it
> > does now.
>
> > --Chouser
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