On Sat, 7 May 2011 12:09:45 -0700
Sean Corfield <seancorfi...@gmail.com> wrote:
> Identity on its own isn't really useful - but in combination with
> higher-order functions, it can be very indispensible!
Bingo. An HOF accepts a function that filters/mogrifies data before
processing it in some way. Sometimes, you *don't* want to have that
extra step. There are two ways to do that: one is to have two variants
of the HOF - one which uses the function, and one which doesn't (which
may mean it's not an HOF). The other is identity.
Clojure does both, depending. You can see the first in sort and
sort-by, where sort-by uses a keyfn to extract keys from items in the
collection. You could just identity for the keyfn to sort by items,
but this case is so common it gets it's own function -
sort. Similarly, filter takes a predicate to check which items need to
be removed. If you just want to remove false values, the appropriate
predicate is identity. This case isn't very common, so there's no
second version.
<mike
--
Mike Meyer <m...@mired.org> http://www.mired.org/
Independent Software developer/SCM consultant, email for more information.
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