difference between first & peek; rest & pop

[Seven Hong]

Hi all,

Could some one explain what's the difference between first and peek, rest and pop? For me it looks like they behave exactly same on sequences..

Thanks!

Best,

Seven Hong

[Michał Marczyk]

They are equivalent when operating on lists, but not so when operating
on vectors (peek = constant time last, pop = log time remove last
element) or queues (peek = first, pop = remove first element returning
a queue -- rest/next would return a seq).

Also, they refuse to work with non-list sequences (try (peek (range))
or (peek (cons :foo ())) at the REPL).

In general, they operate on clojure.lang.IPersistentStack instances.
(That's right, persistent queues implement IPersistentStack.)

Cheers,
Michał

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[Ethan Brooks]

Also, pop throws an exception on the empty list whereas rest returns ().

[Justin Smith]

first and rest are defined in terms of position, and work on anything that can be treated as an ordered collection

peek and pop work in terms of "natural insertion order" and only work with things that behave like a stack - (so not lazy-seqs, strings, etc.)

lists push and pop from the front, vectors push and pop from the end, queues push to one end, pop from the other

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[Gary Verhaegen]

Using a dynamic language doesn't mean you should not think in terms of (abstract) types; it only means that you're note relying on the compiler to check types for you. If anything, that should encourage you to be more disciplined in your thought processes (as you don't have anything else to rely on).

first, rest, next and conj are part of the sequence abstraction; use them when you're logically manipulating a sequence.

peek, pop and conj can be part of either the stack abstraction (if used on lists or vectors) or the queue abstraction (if used on persistent queues).

Because Clojure is highly polymorphic, some names, like conj, can be (and are) reused across abstractions, because they have very similar meanings (e.g. "add something" for conj). You still have to know what context you're using them in in order to understand exactly what they're going to do (e.g. conj on a vector or on a seq across the same vector will not do the same thing).

On 10 November 2017 at 12:49, Justin Smith <noise...@gmail.com> wrote:

first and rest are defined in terms of position, and work on anything that can be treated as an ordered collection

peek and pop work in terms of "natural insertion order" and only work with things that behave like a stack - (so not lazy-seqs, strings, etc.)

lists push and pop from the front, vectors push and pop from the end, queues push to one end, pop from the other

On Thu, Nov 9, 2017 at 6:06 PM Ethan Brooks <ethana...@gmail.com> wrote:

Also, pop throws an exception on the empty list whereas rest returns ().

On Thursday, May 30, 2013 at 12:43:14 AM UTC-4, Seven Hong wrote:

Hi all,

Could some one explain what's the difference between first and peek, rest and pop? For me it looks like they behave exactly same on sequences..

Thanks!

Best,

Seven Hong

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