What is the purpose of the Identity function?

[Base]

At the risk of sounding completely dense, I am having a hard time
understanding the purpose of the Identity function. As far as I can
tell all it does is return what is passed to it.

Review code I see it used all the time, and cannot understand why it
is needed.

Any insight?

Thanks

Base

[Alfredo]

My two cents:
You can use it with the operator ->, in order to pass something to
another function, or for propagating an input of some sort.

It sounds sensed?
Bye,
Alfredo

[Ulises]

I usually use identity as a predicate for functions such as filter,
drop-while, take-while, etc.

Consider this silly example: imagine you had an operation that fetches
stuff from a resource (DB, network, etc.) and that upon failing it
returns nil. Additionally, imagine that you're interested in running
this operation for several resources and keeping those values which
didn't fail. You can do so with identity:

user=> (defn my-operation-that-might-fail [x]
(if (= x "foo") x nil))
#'user/my-operation-that-might-fail
user=> (def some-values ["bar" "foo" "baz"])
#'user/some-values
user=> (filter identity (map your-pred some-values))
("foo")
user=>

and you have your single operation that didn't fail.

My 0.5cts.

U

[Ambrose Bonnaire-Sergeant]

I just grepped the clojure source code and an interesting use is in walk.clj

https://github.com/clojure/clojure/blob/master/src/clj/clojure/walk.clj#L62

`walk` has flexibility with higher order functions, but identity helps with the simple

case of just returning the forms elegantly.

I thought it was cool :)

Ambrose

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[Meikel Brandmeyer]

Hi,

you can also use it to do funny stuff with juxt.

(map (juxt identity f) some-seq)

Sincerely
Meikel

[Sean Corfield]

On Sat, May 7, 2011 at 8:51 AM, Base <basse...@gmail.com> wrote:
> At the risk of sounding completely dense, I am having a hard time
> understanding the purpose of the Identity function.  As far as I can
> tell all it does is return what is passed to it.

It's most useful when you have functions that take functions as
arguments. For example, I have code that performs a SQL query and then
runs a map-reduce transformation on that. Sometimes, however, I want
just the original data so I can pass in identity (to map) and have it
be a "no-op".

Identity on its own isn't really useful - but in combination with
higher-order functions, it can be very indispensible!
--
Sean A Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/
World Singles, LLC. -- http://worldsingles.com/
Railo Technologies, Inc. -- http://www.getrailo.com/

"Perfection is the enemy of the good."
-- Gustave Flaubert, French realist novelist (1821-1880)

[Base]

Ahhh -

Thanks all. Most educational! This does make sense - I will try and
deconstruct some of the examples where this is used to get a sense of
when / why it is used. But this helps *tremendously*!!

Thanks!!!


On May 7, 2:09 pm, Sean Corfield <seancorfi...@gmail.com> wrote:

> On Sat, May 7, 2011 at 8:51 AM, Base <basselh...@gmail.com> wrote:
> > At the risk of sounding completely dense, I am having a hard time
> > understanding the purpose of the Identity function.  As far as I can
> > tell all it does is return what is passed to it.
>
> It's most useful when you have functions that take functions as
> arguments. For example, I have code that performs a SQL query and then
> runs a map-reduce transformation on that. Sometimes, however, I want
> just the original data so I can pass in identity (to map) and have it
> be a "no-op".
>
> Identity on its own isn't really useful - but in combination with
> higher-order functions, it can be very indispensible!
> --
> Sean A Corfield -- (904) 302-SEAN
> An Architect's View --http://corfield.org/
> World Singles, LLC. --http://worldsingles.com/

> Railo Technologies, Inc. --http://www.getrailo.com/

[Mike Meyer]

On Sat, 7 May 2011 12:09:45 -0700
Sean Corfield <seanco...@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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