rob-brown / monadex Goto Github PK

I was wondering if it would still be idiomatic to pipe the monad's value to the right hand side function similar to the pipe operator upon a successful result monad instead of calling the fn closure and passing the value?

def my_func(value), do: IO.puts value

success(value) ~>> my_func

instead of

success(value) ~>> fn value -> my_func(value) end

Dear Rob,
I was wandering how to make a pure functional Monad to use with CQRS. The idea I found here: https://github.com/slashdotdash/eventsourced
Every input command should generate an Event, and this event should be applied on the State. Maybe you could add in MonadEx or illuminate my way to implement it alone.

Hi Daniel,

many thanks for the fantastic Monadex lib, I finally feel that I understood the
monads abstractions, and how important they are to reduce our codebase.
But I still have some questions about the practical use of MonadEx.
Let me ask you, for the "Result" monad, the solution below, to deal with errors during the pipe execution, lacks something to drive me to use MonadEx, or it's enough ? What would be the advantage to use MonadEx over the solution below ?

thanks

Henry

try do
  opts
  |> Keyword.fetch!(:source_file)
  |> File.read!
rescue
  e in KeyError -> IO.puts "missing :source_file option"
  e in File.Error -> IO.puts "unable to read source file"
end

My experience in other languages that support monads, is that you need a transformer to compose monads (eg MaybeT would allow you to cut through a Maybe inside another Monad by wrapping the whole thing in the transformer.)

I tried some time ago to implement transformers for MonadEx, and I hit a wall I believe attempting to represent a higher-kinded type (MaybeT would take some monad, without the contents known, so like MaybeT[Reader[_], A], rather than MaybeT[Reader[X], A].) Sorry if this sounds off, it was a pretty long time ago and I unfortunately didn't save my progress.

Anyway all of that said, my question comes back to the README for MonadEx. There's a heading that talks briefly about composing monads, but no example. Can you clarify how you would go about doing this in Elixir using this lib?

Background

I am trying this library and in specific I am trying the Result Monad. The documentation mentions a function called Result.bind and I have made it work with the following code snippet:

defmodule TestMonadex do
  use Monad.Operators

  import Monad.Result

  def p1(x) do
    x
    |> success()
    |> bind(&plus_1/1)
  end

  defp plus_1(n), do: n + 1
end

And so far so good.

Problem

The problem arises when I try to use the bind operator. According to the docs, this operator is the same thing as the Result.bind function. Following this logic, the following code snippet should work:

defmodule TestMonadex do
  use Monad.Operators

  import Monad.Result

  def p2(x) do
    success(x) ->> (&plus_1/1) # This does not compile!
  end

  defp plus_1(n), do: n + 1
end

Unfortunately, this is not compiling:

(SyntaxError) syntax error before: '>'

Stacktrace:
  │ (elixir 1.11.2) lib/kernel/parallel_compiler.ex:314: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/7

an expression is always required on the right side of ->. Please provide a value after ->

At this point I strongly believe I am not understanding something from the docs or from the library.

Question

What a I doing wrong?

Background

In my quest to learn more about this library I am trying to chain a Result Monad several times in a function.
I understand this is achieved via the fmap, a function which takes a function and a Monad and returns a Monad.

defmodule TestMonadex do
  use Monad.Operators

  import Monad.Result

  # This wont work
  def p2(x) do
    x
    |> success()
    <|> (&plus_1/1)
    <|> (&plus_1/1) 
  end

  defp plus_1(n), do: n + 1
end

Problem

The problem here is that according to the documentation, Result Monad does not implement the fmap:

https://hexdocs.pm/monadex/Monad.Result.html#content

Question

• Why can't my example work?
• How can I chain the Result Monad?