måning

Morning

mogge 😼

:yawn: Morning 😪

got a cold?

Morning friends

Bore da pawb

morning

o/

o/

Nearly got my ClojureX slides done, just a few bits to tighten.

@jasonbell can I get a preview?

just to prepare me...

nope

There’s no way I’m giving you a heckling headstart @thomas

"boo kafka, boo python, that concludes my presentation, thank you everybody for coming"

puts up slide that just says

And the title of my talk is @alex.lynham? 🙂

🎉 💨〰️ 🧠

that's my best emoji rendition of deep learning with kafka and clj, haha

🙂

Not bad 🙂

\o

Hmm

Anyone got dynamodb querying fu here?

Morning :)…

i've got c* query-fu @carr0t... which may or may not be applicable, depending - what's your problem ?

I've got a load of data in a DDB, and some of the import was corrupt, but I don't know which bits

I know the effect, which is that a partition key in DDB will be missing a certain record

With the key 'entity-type' set to the value 'version'

So I want to return all partition keys which don't have a 'version' record

is the table moderate sized enough that you can do a full-table scan ?

that would be my first port of call for a similar c* problem

hmm - and even to build an index table of rows which don't have a particular attribute would require a full-table scan anyway, certainly in c*

~45GB

S'OK, we think we've got a halfway house that won't necessarily fix all the data, but it'll fix the bits we care about 😉

Hey now, hey now, don't dream....

But (+) is 0

Yet (-) is not defined.

Kind of wanted the minus to have an identity. And I'm suprised it doesn't seem to.

Anyone know why?

user=> (doc -)
-------------------------
clojure.core/-
([x] [x y] [x y & more])
  If no ys are supplied, returns the negation of x, else subtracts
  the ys from x and returns the result. Does not auto-promote
  longs, will throw on overflow. See also: -'
nil
user=> (doc +)
-------------------------
clojure.core/+
([] [x] [x y] [x y & more])
  Returns the sum of nums. (+) returns 0. Does not auto-promote
  longs, will throw on overflow. See also: +'
nil

Interesting.

Probably because subtraction isn't commutative?

true, but zero is still the identity

(defn vector_arith [f]
  (fn
    ([]            [(f) (f)])
    ([[x y]]       [(f x (f)) (f x (f))])       
    ([[x y] [a b]] [(f x a) (f y b)])
    ))

I wanted to be able to pass in - or + to convert them to work over vectors

given that - works on any arity >=1 and there is an identity it would seem reasonable for it to also work with arity 0

@mattford no it isn't

0 is the right identity of -, but it's not its identity

>Let (S, ∗) be a set S with a binary operation ∗ on it. Then an element e of S is called a left identity if e ∗ a = a for all a in S, and a right identity if a ∗ e = a for all a in S. If e is both a left identity and a right identity, then it is called a two-sided identity, or simply an identity.

ah okay

(transduce identity + [1 2 3])
6
witan.send.model.run> (transduce identity - [1 2 3])
ArityException Wrong number of args (0) passed to: core/-  clojure.lang.AFn.throwArity (AFn.java:429)

a bit annoying

user=> (transduce identity - 0 [1 2 3])
6

yeah but I now need to know the identity

or not the identity 🙂

the right-identity

we can't change how maths work in order to make a particular use-case more ergonomic :)

can it be the case that we use the right-identity for clojure though?

no

transduce does

that's a particular use-case

as long as you don’t call it a monoid

if you were to fold right instead of left it would be broken

ah yeah

Cheers, back to work.

(actually it's the other way round)

you can put your function in to test check to check if it passes the laws https://gist.github.com/minimal/40085951c277d8ce26ad

transduce would need the left identity

@bronsa... surely changing math just requires a PR?

which doesn't exist for - !

🚪 🏃

@minimal I like that monoid checker 🙂

monoids are really useful

thanks, I think I probably stole the initial check from somewhere and added to it