[erlang-questions] Refactoring in anger

Felix Gallo felixgallo@REDACTED
Sun Aug 9 04:06:58 CEST 2015

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.


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.
>>> _______________________________________________
>>> erlang-questions mailing list
>>> erlang-questions@REDACTED
>>> http://erlang.org/mailman/listinfo/erlang-questions
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