notes.txt

Scala School notes. Marius & Steve (Aug 25, 2011)

notes

  - slim searchbird.tar.gz, make available, put on front page

lesson 1:

* cases are introduced too early (in try{}catch{}) (moved to end of lesson 2)
* bootstrap vs. screen width (not sure how to fix)

lesson 2:

* you can leave out parenthesis (explained in lesson 1)
* var vs. val (explained in lesson 1)
* why default case, runtime exceptions (fixed)
* currentCount should have parens (fixed)
* introduce traits too early (introduced traits to Lesson 1)
* remove .equals (fixed)
* match … Bottle(thisColor, theseOunces) (fixed)
* don't omit parens for partial application (fixed)
* java.lang.Object in function (switched to use an object extending Function1)
* motivate case classes better - structs (fixed)
* defer case classes until after matching (maybe) (show that it would be painful to do the blue bottle example without case classes) (fixed)

* introduce blocks? (lesson 1)
* introduce casting
* module system
* didn't explain brackets (TODO: should we have a mini-types explanation earlier?)

lesson 3:
* todo: introduce variable length arguments (Lesson 1)
* get rid of recap (fixed)
* introduce tuples (already there)
* kill Seq (fixed)
* flatMap (fixed)
* note that these are combinators -- transformations on collections (fixed)
* introduce currying (in lesson #1)
* kill the block in foldLeft (ok, fine. fixed)
* index vs. value is confusing (where is this?)
* Map(1 -> 2) looks like special syntax (fixed)
* find better example for partial application (fixed: add(m)(n))
* compositional semantics of currying argument lists vs. partial application, vs. …

lesson 4:

* don't use whitespace application
* move composition all the way up
* introduce option!
* show case classes after unapply
* introduce options together with collections (collections of 1 or 0)
* PartialFunction != partially applied function! (fixed)
* introduce whitespace applicaton of methods
* change wildcard partial function to a total function
* for partial functions, don’t say ‘closed, say ‘is only defined for certain arguments’

* introduce Option earlier

(Steve’s half took 1.5 hours)

Feedback: can we show more code?  larger examples of matching, for
example.

Map(1 -> "a") is confusing.  special syntax?  syntactic sugar?
(Fixed)

Explain types earlier.

Font is too light for a projector.  Important things should be bold.
Font is too small, border too large.  Great for a screen, bad for a
projector.  (Toggle a style-sheet?)

Lesson: Types

Q: What do we need for finagle?

Idea: the example functions should be based on searchbird so we can
later stitch together functions we've already built and understand.

Note from Sam List[Any] example makes you think it'll always resolve
to Any but it will be most common super-type.

scala> trait F defined trait F

scala> class A extends F defined class A

scala> class B extends F defined class B

scala> List(new A, new B) res55: List[ScalaObject with F] =
List(A@1bac8b63, B@5cf31ec6)

Feedback: aren't showing how to encode higher-rank polymorphism but
talk about it.  why do that?

Feedback: should we discuss HM since Scala doesn't do it?  Can we
explain the ad-hoc rules more easily?

{ x => x } shows {} use but we haven't introduced that yet.

In Types, we use return type inference heavily but nowhere else.
(should we explain and use return type inference earlier?)

Feedback: AnyRef has made an appearance but we haven't explained it
yet.

We show trait Function but haven't explained a trait yet.

We use the function value syntax but generally only show partially
applied functions earlier so is confusing.

val fB: (A => B) = (a: A) => new B(a)

vs

def fB(a: A): B) = new B(a)


Feedback: make it non-mathematical, don't use A, B, C, use Animal,
Cats and Dogs rather than more abstract.

Invariant isn't covered and yet is the default so people will always
need to understand it.

note: functions are contravariant on their arguments.

this would be clearer as animals, with a pound or cage method?

We often explain things in person that we then explain things in the
talk.  (need more practice)

wildcard not needed for finagle so skip.

Lesson: Finagle (took 20 minutes to get here)

we need to add flatMap earlier.

Finagle lesson uses Java callback as an example: should switch to
Scala.

we use for-comprehension syntax but haven't explained that yet.  with
yield.  in the collections/combinators lesson.

...  looks like an underscore (add spacing)

def Future[A].flatMap(...) is odd looking syntax.  (is for
illustration).  put it in a trait:

trait Future[A] {
  def flatMap[B](f: A => Future[B]): Future[B]
}

Q: "why is it called flatMap?" (haven't described collections or
for-comprehensions so people don't know the contract)

failures short-circuit sequential composition.

We haven't explained Unit yet.

We use dot-less syntax but haven't explained that yet.

Two uses join methods: one returns Future[Unit], the other doesn't.

`dispatch(request) join userLimit` is confusing and controversial
because dispatch is always called.

Not stock _or_ combinator.  can write it with flatMap.

handle is flatMap for errors.

error-handling, generally, should be made more clear: the fact that
it's threaded everywhere should be called out.

Thought: get laser pointers.  (also bring our own VGA/DVI adapters)

use function extension

class Foo extends (A => B) but haven't explained that yet.

Remove Passbird from Finagle example.

Feedback: call out the type-safety of authenticatedTimedOutService in
text.

Filter brings up the question of "what's blocking?  can I block here?"


Lesson: Searchbird (have 25 minutes left)

Need a way to hide the examples we want people to fill out.

sbt transcript

sbt should be ./sbt (to make sure we use our local sbt) skip update,
don't do that!

don't make people do a gem install of thrift_client.

mine fails.

System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in
`gem_original_require': no such file to load -- bundler/setup
(LoadError)
	from
	/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in
	`require' from src/scripts/console:5

d'oh.

turn this into a scala client, show people finagle clients.

ServerConfig is hard to explain.

"RuntimeEnvironments has command-line flags, and other process
information"

Future[Void] is in the trait (we still haven't explained traits)

In the Implementation scala code:

concurrency bug to not sync around the map.

Future(value) should be Future.value(value) // d'oh, value twice

database(key) = value

motivates explaining update syntax sugar (or else not do that in the
implementation)

forward/reverse hashmaps use mixins.

Why is there Future.void but not Future.unit?

we use a reduceLeftOption and then getOrElse without dots.

Sub-lesson: "Distributing our service" (have 10 minutes left)

we use require(!indices.isEmpty)?

there's an example of finagle clients in here that we can pull out
into an object for a simple repl-based client.

val Array(_, port) = shards(which).split(":")

haven't explained the deconstruction syntax here.

Idea: thumb drives with the code.

¶

notes from Dave L. re: searchbird lesson

When you add the search method to the thrift IDL, it would be nice to
highlight just the added line.

"excercise" -> "exercise"

Would be cool to have an "Exercises" section at the end for all the
go-getters to work through.

Am I having a brain fart, or should these be made consistent (meaning
use the 2-argument getOrElse in both places)?
   val current = reverse.get(token) getOrElse Set()
vs.
   val hits = tokens map { token => reverse.getOrElse(token, Set()) }


More ideas for followup exercises:

- flesh out put() on CompositeIndex to have a scheme for sending put
requests to specific shards (round-robin? some attribute of the
document?)

- the search() method performs an intersection of the documents
matching the terms (an AND query). implement a union (OR query).

- the result set size for search() is unbounded.  add support for a
default result size, and the ability to paginate through the full
result set.  think about the expense of this operation on large result
sets, and how you might optimize the system for this.

That's all for now... I'd love to go through the full set of material,
but can't get to it tonight.  But I'm really looking forward to
incorporating this into NHO.  If things go well tomorrow, perhaps I
can try it out on the current crop of newbies?