[erlang-questions] Refactoring in anger (Felix Gallo)

Roelof Wobben r.wobben@REDACTED
Sun Aug 9 10:31:44 CEST 2015

Op 9-8-2015 om 8:25 schreef J David Eisenberg:
> Felix Gallo wrote:
>> Nevertheless, if you're going to level a thundering, public j'accuse at
>> someone who has clearly gone to great effort to provide a beginner
>> experience for erlang newbies, without (apparently) first contacting him
>> privately and suggesting improvements in a constructively critical manner,
>> please have the decency to run the code that you have etched into your
>> stone tablets *before* you hold them aloft and, with fiery mane ablaze in
>> the evening sun, present them as the replacement for the gentleman's work.
>> There are many "problems" with the etudes code.  I'm not sure that
>> introducing a complex exception handling workflow is one of them, to be
>> frank, pedagogically, at the moment that the student is trying to
>> understand pattern matching and function headers.  Could be.  But I think
>> we can say it's a matter of taste.
>> And that's what really rubs me the wrong way about Garrett's post.  Being
>> 'very preachy', even when volubly disclaimed, is still pretty tasteless.
>> But being 'very preachy' and then slapping up code you haven't even run
>> once, directly in opposition to your preaching's core point: that's
>> hypocritical, and super tasteless.  And I enjoy my rich dark ironic comedy
>> as much as the next guy, but come on.
>> F.
> Author of Etudes here.
> There I was at Pepe's Tacos in Las Vegas, waiting for my meal, and I decided
> to read the Erlang questions mail list and see what's going on. As the story
> unfolded, I started cackling with laughter, until I realized that half
> the people
> in the restaurant were staring at me like "What the hell is wrong with this
> crazy white guy laughing at his phone?"
> So no, I'm not even remotely offended. Especially because I write in
> the preface:
> "I was learning Erlang as I was creating the solutions to the études,
> following the philosophy that 'the first way that works is the right way.'
> Therefore, don’t be surprised if you see some fairly naïve code that an expert
> Erlang programmer would never write."
> Since the book is open source, if anyone would like to add a better
> solution with
> a discussion of why it's better, I would be perfectly happy to see that. Modulo
> the anger, of course :)
>> On Sat, Aug 8, 2015 at 6:19 PM, Leandro Ostera <leandro@REDACTED> wrote:
>>> Just a minor typo: Line 48 should is trying to reassign the Prompt
>>> variable.
>>> But +1 on the rest.
>>> On Sun, Aug 9, 2015 at 2:36 AM, Felix Gallo <felixgallo@REDACTED> wrote:
>>>> Eshell V7.0  (abort with ^G)
>>>> 1> c(geom).
>>>> {ok,geom}
>>>> 2> c(ask_area).
>>>> {ok,ask_area}
>>>> 3> ask_area:print_area().
>>>> R)ectangle, T)riangle, or E)llipse > T
>>>> ** exception error: no function clause matching
>>>> ask_area:error_msg({badmatch,["Enter ","base"," > "]}) (ask_area.erl, line
>>>> 69)
>>>>       in function  ask_area:print_area/0 (ask_area.erl, line 9)
>>>> glass houses, etc.
>>>> F.
>>>> On Sat, Aug 8, 2015 at 3:07 PM, Garrett Smith <g@REDACTED> wrote:
>>>>> We've had an ongoing thread on how to handle some problems
>>>>> idiomatically in Erlang:
>>>>> http://erlang.org/pipermail/erlang-questions/2015-August/085382.html
>>>>> The OP's questions are coming out of an exercise here:
>>>>> http://chimera.labs.oreilly.com/books/1234000000726/ch05.html#CH05-ET01
>>>>> This in turn points to a proposed solution here:
>>>>> http://chimera.labs.oreilly.com/books/1234000000726/apa.html#_literal_geom_erl_literal_9
>>>>> Upon reading this solution, I became so enraged [1] that I rewrote the
>>>>> module and want to make a number of points.
>>>>> Here's my rewrite:
>>>>> https://gist.github.com/gar1t/7bb80d728f804554ac32
>>>>> The tone of my points below is *very preachy* which is going to annoy
>>>>> some people here. I apologize in advance - but hey, ranters gotta
>>>>> rant.
>>>>> # Clarity of "what's going on"
>>>>> Compare my area/0 to the original area/0. Which is easier to see
>>>>> "what's going on"? I'm not boasting here but rather making the most
>>>>> important point I can about programming: take the time to be clear in
>>>>> your intent! If you're actually clear - in your brain - making the
>>>>> code reflect your understanding *is not that hard*. If your code is
>>>>> not clear, chances are your brain is not clear.
>>>>> Maybe the original code works, maybe it doesn't. I can't tell from
>>>>> looking at that function, at all. I have to dig around various
>>>>> implementation details and hold a bunch of information in my brain to
>>>>> understand what the author is maybe trying to do. At some point I
>>>>> can't keep it all straight and have to run the thing and observe its
>>>>> behavior. *Now* I'm thinking, what happens to the pour schlep who has
>>>>> to modify this code. In the real world, this causes fear, and
>>>>> loathing, and tests - lots and lots of tests!
>>>>> We don't have to live this way.
>>>>> # Separation of concerns (in this case IO vs calculations)
>>>>> The original calculate/3 mixes user facing behavior (print error
>>>>> messages) with a calculation. If your function returns a mishmash of
>>>>> completely unrelated values (e.g. the result of io:format and also the
>>>>> result of an area calculation) *it's a mess*.
>>>>> In the rewrite I've added print_area/0, which is responsible for
>>>>> displaying results to the user. area/0 returns an area or raises an
>>>>> exception. print_area/0 handles both the result and any error.
>>>>> # Handling invalid input
>>>>> The original thread here involves a discussion on how to handle bad
>>>>> input to a function. My rewrite does one of two things on this front:
>>>>> - If input is from a user, there's an explicit exception raised on bad
>>>>> input that can be used by an error handler to inform the user
>>>>> - If input is not from a user but rather internal, I don't handle bad
>>>>> input at all, but let Erlang crash with a function or case clause
>>>>> error
>>>>> The first case address the user experience. We could let exceptions
>>>>> just propagate and upset users with arcane Erlang messages. Or we can
>>>>> handle errors politely with intelligible messages.
>>>>> The original ask_erl.erl handles invalid input by passing atoms like
>>>>> 'error' and 'unknown' along a call chain. This is tedious and finally
>>>>> culminates in the original calculate/3 - a monster mishmash function
>>>>> of error handling, IO, and calculation.
>>>>> My rewrite raises an exception for those functions that take user
>>>>> provided input. I prefer exceptions in this case as they keep return
>>>>> values on the happy path, which makes code easier to read.
>>>>> I don't care about handling internal errors, as long as they manifest
>>>>> in an obvious way.
>>>>> # Avoid variable assignment/binding inside case expressions
>>>>> Just don't do this:
>>>>>    case Shape of
>>>>>        rectangle -> Numbers = get_dimensions("width", "height");
>>>>>        triangle -> Numbers = get_dimensions("base", "height");
>>>>>        ellipse -> Numbers = get_dimensions("major axis", "minor axis")
>>>>>    end
>>>>> Now that you're not allowed to do that, what? Hey, a function!
>>>>>    Numbers = get_dimensions(Shape)
>>>>> Every time, all the time.
>>>>> # Consider not using case expressions at all
>>>>> Think of a function as a named case expression with a well defined scope.
>>>>> You'll find that having to name the function forces you to think about
>>>>> "what's going on" there. It will help your reader (often that means
>>>>> you, later on) to understand your intention.
>>>>> My rewrite doesn't use a single case expression. Is it somehow worse?
>>>>> It's better!
>>>>> # If it looks confusing, it's bad!
>>>>> Erlang is not C, or bash, or Perl, or Ruby. It's possible to write
>>>>> really easy-to-read code in Erlang. People who complain about Erlang
>>>>> syntax are probably complain about terrible code in Erlang. Terrible
>>>>> code in any language is worth complaining about - but it's unrelated
>>>>> to syntax.
>>>>> It's easy to spot bad code in Erlang:
>>>>> - Long functions
>>>>> - Excessive nesting (more than two levels is a train wreck, and IMO
>>>>> more than one is bad)
>>>>> I hate nesting so much that I'll go to the trouble of writing
>>>>> to_number/1 as a list of "try" attempts (see rewrite). Some people
>>>>> call this monadic, which I like because it sounds super cool.
>>>>> - Variable assignment/binding inside case and if expressions (see above)
>>>>> - Functions that are a litany of imperative style instructions, like
>>>>> this:
>>>>>    get_number(Prompt) ->
>>>>>        Str = io:get_line("Enter " ++ Prompt ++ " > "),
>>>>>        {Test, _} = string:to_float(Str),
>>>>>        case Test of
>>>>>            error -> {N, _} = string:to_integer(Str);
>>>>>            _ -> N = Test
>>>>>        end,
>>>>>        N.
>>>>> This fails the "long functions" test - but lines per function is just
>>>>> a proxy. The real problem here is that it takes too much effort to
>>>>> read and understand. Really, this is an imperative pattern applied
>>>>> naively to Erlang. No!
>>>>> Try this:
>>>>>    number_from_user(Prompt) ->
>>>>>        to_positive_number(prompt_user(["Enter ", Prompt, " > "])).
>>>>> Where's the rest of the code? It's there, inside the other functions.
>>>>> But *this* function doesn't make you deal with that detail because it
>>>>> wants you to understand what *it* is doing.
>>>>> Okay, so I've been pretty critical here of this online training
>>>>> resource, which some will consider bad form. So let's turn this into
>>>>> something nice and kind!
>>>>> The subject of this email is "refactoring in anger", which turns out
>>>>> to be something I routinely do with my own code. It's hard to write
>>>>> the perfect code in one pass. So I tend to just "get it to work" and
>>>>> then examine what seems to be working very carefully - and then change
>>>>> it so that it becomes *super obvious* what's going on. This state can
>>>>> take a few passes and still might have warts. Okay, no sweat - it's
>>>>> just code. Nothing to get angry about. Calm down and fix it.
>>>>> Over time, when you practice this pattern of refactoring, you'll start
>>>>> writing things clearly the first time. That's because you'll start
>>>>> *thinking* more clearly the first time.
>>>>> Now that's positive and nice and kind right?
>>>>> Garrett
>>>>> ---
>>>>> [1] Of course I'm joking here, but only partly. The original solution
>>>>> is presented to learners - and it's full of bad practices, so that
>>>>> makes me cranky. I suppose it's good to have teaching material for
>>>>> Erlang in any form - at least it serves as a point of discussion, even
>>>>> if contentious.
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I was the person who was looking for help because I was more then 1 week 
busy with this exercise.
I understand the solution presented by Garret. The only things I miss 
are the documentation of the functions. (spec and explanations).


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