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Dabbling in Erlang, Part 2: A Minimal Introduction (kontext.gr)
75 points by pasxizeis on Oct 12, 2013 | hide | past | favorite | 17 comments

In the section "Functional programming for real" he states:

> Note that the order of the definitions is important: had we moved our base case at the bottom, the recursion would never finish because map(F, [H|T]) would always match (remember? an empty list is also a list with a head and a tail).

Which confuses me a bit, since I am used to the empty list not having a head (I am thinking for example of Haskell). Is it true that in Erland the empty list has a head? And what would it be? Some generic null object?

He is right in that order is important when pattern matching. However, his premise - an empty list has a head and a tail - is incorrect, so in this case order does not matter:

  > [H] = []. 
    ** exception error: no match of right hand side value []
  > [H|T] = [].
    ** exception error: no match of right hand side value []
But when it comes to pattern matching, order is important:

  > Test = fun(Value) -> Value ; (hello) -> world end.
  > Test(hello).
As opposed to:

  > Test2 = fun(hello) -> world ; (Value) -> Value end.
  > Test2(hello). 

For the sake of completeness it's worth mentioning that a list with a single item has both a head and a tail:

  > [H|T] = [1]. 
  > H. 
  > T. 
(Which is why you need to have a function head that matches [], whether first or last in his example.)

For readability, it's nice to have the base case at the top. For performance (if performance is an issue) it's nice to have the base case at the bottom since all function heads are evaluated in order.

If you're looping over a million element list and your base case is first, it'll be checked a million times. If your base case is last, it'll only be checked once when your normal cases no longer match.

And to clear up your confusion, the writeup is wrong about empty lists matching on [H|T]:

   Eshell V5.10.1  (abort with ^G)
   1> [H|T] = [1, 2, 3, 4, 5].
   2> [H2|T2] = [].
   ** exception error: no match of right hand side value []

The BEAM virtual machine is able to reorder most clauses based on need, and for most cases, a binary search will be done through all clauses. Exceptions include function clauses with guards, which cannot easily be reordered but still impact whether a clause will be used or not.

The VM will instead try to reorder the clauses up until the next guard clause, if possible.

You should mostly care about writing readable code and let the compiler and the VM do the rest.

Yep, I was wrong (fixed the post).

The empty list does not have a head, neither a tail, it's just an empty list.

I've replaced it with another example. Thanks for all the feedback by the way.

No, he is wrong in this case. It would work fine. What he probably meant to show is that a list with a single item also has a tail. Meaning if you wanted to do:

map(F, [H]) -> [F(H)]; map(F, [H|T]) -> [F(H) | map(F, T)].

Order does matter

I have written a more quickly paced Erlang tutorial some years ago, it is not even online anymore, but here is a copy from archive.org:


I must shamelessly say it seems to have aged reasonably well, unlike some other things I wrote I actually read it now with interest, especially I haven't dabbled in Erlang since and forgot some details.

I recommend everyone who finds fun in programming to give Erlang a shot, I don't think it's great for all the use cases (what is?), but, for example, for writing servers of all kinds it's totally awesome, especially with OTP, which I didn't cover in my tutorial. It's also completely different from everything else, even if you know Prolog or Haskell/ML, which are related in some ways. One day I would really like to get myself together and write something like a game server for poker or bridge in it.

This is a pretty good introduction and is pretty accessible on first pass through.

However, a factual error:: "(remember? an empty list is also a list with a head and a tail)."

An empty list is an empty list :) You could switch the order of those two function heads and 'map' would still behave properly.

Yes that was a mistake I made. I've fixed it now thanks to your feedback, with a proper example.

For anyone finding the syntax of list comprehensions a bit confusing (like me), I found this comparison to be useful if you're familiar with the more readable (usually) Python equivalent:

    [expression(element) || element <- list, conditions]
is the same as

    [expression(element) for element in list if condition]

Is there some platform that offers such a VM as Erlang's one, but without the functional language ?

Erlang VM is built on the premise that it runs a functional language. I would argue that it will be extremely inefficient to implement non-function language to run on Erlang VM. By non-functional I mean language that is based on mutable and shared state. You can look at Elixir (http://elixir-lang.org/) to see great example of an alternative language that runs natively on Erlang VM. While offering syntax that is more familiar to many people Elixir retains all basic restrictions of Erlang.

That's awesome mate keep up!!!

I like the pace, the formatting, and the content. Would love to see more! Great work!

Hey, thanks!

Two .gr / Greek links on HN in a day, nice.

One of main contributors to Erlang is Kostis Sagonas from Athens National Technical University. He is one of the main contributors to HiPE (the native code compiler), and creator of PropEr (property based tester).

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