Variable -1 OOP approach?

Questions / Help
#1

So, I really new to elixir and starting to use resources on youtube etc.
Working through Live Learning Elixir part 1 @ around 90 minutes…

He adds this and, it compiles.

  def convert_input_to_index(integer_input) do
    integer_input -1
  end

On my machine, it does not. The guy is coming to elixir from ruby but I know, variables, lists etc are immutable right?

I get this error

$ iex "tic_tac_toe.exs"
Erlang/OTP 22 [RELEASE CANDIDATE 2] [erts-10.3.1] [source-52dd4f4842] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1] [hipe]

warning: variable "integer_input" is unused (if the variable is not meant to be used, prefix it with an underscore)
  tic_tac_toe.exs:25

** (CompileError) tic_tac_toe.exs:26: undefined function integer_input/1
    (elixir) src/elixir_locals.erl:108: :elixir_locals."-ensure_no_undefined_local/3-lc$^0/1-0-"/2
    (elixir) src/elixir_locals.erl:109: anonymous fn/3 in :elixir_locals.ensure_no_undefined_local/3
    tic_tac_toe.exs:3: (file)

I now underscore the 'variable _integer_input

Now I get a new error message

$ iex "tic_tac_toe.exs"
Erlang/OTP 22 [RELEASE CANDIDATE 2] [erts-10.3.1] [source-52dd4f4842] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1] [hipe]

** (CompileError) tic_tac_toe.exs:26: "_integer_input -1" looks like a function call but there is a variable named "_integer_input", please use explicit parentheses or even spaces
    tic_tac_toe.exs:25: (module)

Now, is this just the fact he is using an older version and this way is depreciated? Or, is the variable -1 flawed from the start because I understood functional programming required making copies of lists etc and you couldn’t directly modify them as per ruby etc and OOP.

If this approach is wrong, i rather stop trying to fix it now TBH.

Thanks

#2

Due to the fact, that functions and variables live in a separate namespace and the fact that parenthesis are optional, the compiler thinks that you try to call integer_input(-1).

To actually subtract 1 from the input, you need to have a space () between - and 1. Then the parser will realise it as the binary minus, which again then will make it recognize integer_input as the variable rather than a function name.

Also, the error:

warning: variable "integer_input" is unused (if the variable is not meant to be used, prefix it with an underscore)
  tic_tac_toe.exs:25

Wanted you to prefix the name in the functions argument list, not its usage in the functions body, eg: def foo(_unused), do: …

PS: I’m not sure what this has to do with “OOP”?

4 Likes
#3

I will try that thanks.

The guy who did the video is a rubyist and he is explaining everything from an OOP perspective.
This is why I thought var - 1 was as illegal as var++.

It’s crazy that a space is needed. Something I haven’t read about yet so thanks.

#4

The parser needs that space to distinguish unary and binary minus, it comes up here every now and then :wink:

And why should var - 1 not be valid? It creates a new value and leaving var as is. Nothing different to most OO languages (unless some very nasty stiff has been done via operator overloading).

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#5

Yes, one space away from a compile. I can’t believe formatters aren’t alert to this.

Lesson learned. Many thanks.

#6

My misunderstanding. I thought, all lists and values could not be changed, only new copies applied to new values.

#7

You’re correct. This function is not changing var it is simply returning the integer that is one less than var

3 Likes