rpn_calculator
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{
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"authors": [
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"neenjaw"
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],
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"contributors": [
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"angelikatyborska",
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"cjmaxik"
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],
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"files": {
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"solution": [
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"lib/rpn_calculator.ex"
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],
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"test": [
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"test/rpn_calculator_test.exs"
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],
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"exemplar": [
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".meta/exemplar.ex"
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]
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},
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"language_versions": ">=1.10",
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"icon": "instruments-of-texas",
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"blurb": "Learn about errors and rescuing them by working on an experimental Reverse Polish Notation calculator."
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}
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{"track":"elixir","exercise":"rpn-calculator","id":"ea46ce98f99e40f18d385997dfdae1de","url":"https://exercism.org/tracks/elixir/exercises/rpn-calculator","handle":"negrienko","is_requester":true,"auto_approve":false}
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# Used by "mix format"
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[
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inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
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]
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# The directory Mix will write compiled artifacts to.
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/_build/
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# If you run "mix test --cover", coverage assets end up here.
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/cover/
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# The directory Mix downloads your dependencies sources to.
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/deps/
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# Where third-party dependencies like ExDoc output generated docs.
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/doc/
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# Ignore .fetch files in case you like to edit your project deps locally.
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/.fetch
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# If the VM crashes, it generates a dump, let's ignore it too.
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erl_crash.dump
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# Also ignore archive artifacts (built via "mix archive.build").
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*.ez
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# Ignore package tarball (built via "mix hex.build").
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errors-*.tar
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# Help
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## Running the tests
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From the terminal, change to the base directory of the exercise then execute the tests with:
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```bash
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$ mix test
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```
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This will execute the test file found in the `test` subfolder -- a file ending in `_test.exs`
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Documentation:
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* [`mix test` - Elixir's test execution tool](https://hexdocs.pm/mix/Mix.Tasks.Test.html)
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* [`ExUnit` - Elixir's unit test library](https://hexdocs.pm/ex_unit/ExUnit.html)
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## Pending tests
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In test suites of practice exercises, all but the first test have been tagged to be skipped.
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Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.
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For example:
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```elixir
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# @tag :pending
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test "shouting" do
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assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
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end
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```
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If you wish to run all tests at once, you can include all skipped test by using the `--include` flag on the `mix test` command:
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```bash
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$ mix test --include pending
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```
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Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the file `test/test_helper.exs`.
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```elixir
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# ExUnit.configure(exclude: :pending, trace: true)
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```
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## Useful `mix test` options
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* `test/<FILE>.exs:LINENUM` - runs only a single test, the test from `<FILE>.exs` whose definition is on line `LINENUM`
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* `--failed` - runs only tests that failed the last time they ran
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* `--max-failures` - the suite stops evaluating tests when this number of test failures
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is reached
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* `--seed 0` - disables randomization so the tests in a single file will always be ran
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in the same order they were defined in
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## Submitting your solution
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You can submit your solution using the `exercism submit lib/rpn_calculator.ex` command.
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This command will upload your solution to the Exercism website and print the solution page's URL.
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It's possible to submit an incomplete solution which allows you to:
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- See how others have completed the exercise
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- Request help from a mentor
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## Need to get help?
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If you'd like help solving the exercise, check the following pages:
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- The [Elixir track's documentation](https://exercism.org/docs/tracks/elixir)
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- The [Elixir track's programming category on the forum](https://forum.exercism.org/c/programming/elixir)
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- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
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- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
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Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
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If you're stuck on something, it may help to look at some of the [available resources](https://exercism.org/docs/tracks/elixir/resources) out there where answers might be found.
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# Hints
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## General
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- Read about [errors][errors] in the Getting Started guide.
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- Read about [`try`][docs-try] in the documentation.
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## 1. Warn the team
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- Allow the operation function to raise its error.
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- To invoke a function in a variable, use the `.` operator.
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## 2. Wrap the error
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- Make use of try .. rescue to return the intended result.
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## 3. Pass on the message
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- Make use of try .. rescue to return the intended result.
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- The rescue block allows you to pattern match on the error's Module name and also bind the error to a variable.
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[errors]: https://hexdocs.pm/elixir/try-catch-and-rescue.html#errors
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[docs-try]: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#try/1
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# RPN Calculator
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Welcome to RPN Calculator on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
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## Introduction
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## Errors
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Errors happen. In Elixir, while people often say to "let it crash", there are times when we need to rescue the function call to a known good state to fulfill a software contract. In some languages, errors are used as a method of control flow, but in Elixir, this pattern is discouraged. We can often recognize functions that may raise an error just by their name: functions that raise errors are to have `!` at the end of their name. This is in comparison with functions that return `{:ok, value}` or `:error`. Look at these library examples:
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```elixir
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Map.fetch(%{a: 1}, :b)
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# => :error
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Map.fetch!(%{a: 1}, :b)
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# => raises KeyError
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```
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## Try/Rescue
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Elixir provides a construct for rescuing from errors using `try .. rescue`
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[]: # (elixir-formatter-disable-next-block)
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```elixir
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try do #1
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raise RuntimeError, "error" #2
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rescue
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e in RuntimeError -> :error #3
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end
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```
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Let's examine this construct:
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- **Line 1**, the block is declared with `try`.
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- **Line 2**, the function call which may error is placed here, in this case we are calling `raise/2`.
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- **Line 3**, in the `rescue` section, we pattern match on the _Module_ name of the error raised
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- on the left side of `->`:
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- `e` is matched to the error struct.
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- `in` is a keyword.
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- `RuntimeError` is the error that we want to rescue.
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- If we wanted to rescue from all errors, we could use `_` instead of the module name or omit the `in` keyword entirely.
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- on the right side:
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- the instructions to be executed if the error matches.
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### Error structs
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Errors (sometimes also called "exceptions") that you rescue this way are structs.
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Rescuing errors in Elixir is done very rarely.
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Usually the rescued error is logged or sent to an external monitoring service, and then reraised.
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This means we usually don't care about the internal structure of the specific error struct.
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In the [Exceptions concept][exercism-exceptions] you will learn more about error structs, including how to define your own custom error.
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[exercism-exceptions]: https://exercism.org/tracks/elixir/concepts/exceptions
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## Instructions
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While working at _Instruments of Texas_, you are tasked to work on an experimental Reverse Polish Notation [RPN] calculator written in Elixir. Your team is having a problem with some operations raising errors and crashing the process. You have been tasked to write a function which wraps the operation function so that the errors can be handled more elegantly with idiomatic Elixir code.
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## 1. Warn the team
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Implement the function `calculate!/2` to call the operation function with the stack as the only argument. The operation function is defined elsewhere, but you know that it can either complete successfully or raise an error.
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```elixir
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stack = []
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operation = fn _ -> :ok end
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RPNCalculator.calculate!(stack, operation)
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# => :ok
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stack = []
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operation = fn _ -> raise ArgumentError, "An error occurred" end
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RPNCalculator.calculate!(stack, operation)
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# => ** (ArgumentError) An error occurred
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```
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> Function names that end in `!` are a warning to programmers that this function may raise an error
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## 2. Wrap the error
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When doing more research you notice that many functions use atoms and tuples to indicate their success/failure. Implement `calculate/2` using this strategy.
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```elixir
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stack = []
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operation = fn _ -> "operation completed" end
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RPNCalculator.calculate(stack, operation)
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# => {:ok, "operation completed"}
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stack = []
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operation = fn _ -> raise ArgumentError, "An error occurred" end
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RPNCalculator.calculate(stack, operation)
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# => :error
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```
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## 3. Pass on the message
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Some of the errors contain important information that your coworkers need to have to ensure the correct operation of the system. Implement `calculate_verbose/2` to pass on the error message. The error is a struct that has a `:message` field.
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```elixir
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stack = []
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operation = fn _ -> "operation completed" end
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RPNCalculator.calculate_verbose(stack, operation)
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# => {:ok, "operation completed"}
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stack = []
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operation = fn _ -> raise ArgumentError, "An error occurred" end
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RPNCalculator.calculate_verbose(stack, operation)
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# => {:error, "An error occurred"}
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```
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## Source
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### Created by
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- @neenjaw
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### Contributed to by
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- @angelikatyborska
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- @cjmaxik
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defmodule RPNCalculator do
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def calculate!(stack, operation) do
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operation.(stack)
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end
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def calculate(stack, operation) do
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try do
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{:ok, operation.(stack)}
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rescue
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_e -> :error
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end
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end
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def calculate_verbose(stack, operation) do
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try do
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{:ok, operation.(stack)}
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rescue
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e -> {:error, e.message}
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end
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end
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end
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defmodule RPNCalculator.MixProject do
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use Mix.Project
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def project do
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[
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app: :rpn_calculator,
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version: "0.1.0",
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# elixir: "~> 1.10",
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start_permanent: Mix.env() == :prod,
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deps: deps()
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]
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end
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# Run "mix help compile.app" to learn about applications.
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def application do
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[
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extra_applications: [:logger]
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]
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end
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# Run "mix help deps" to learn about dependencies.
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defp deps do
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[
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# {:dep_from_hexpm, "~> 0.3.0"},
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# {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
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]
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end
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end
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defmodule RPNCalculatorTest do
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use ExUnit.Case
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@tag task_id: 1
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test "calculate! returns what the operation does: :ok atom" do
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assert RPNCalculator.calculate!([], fn _ -> :ok end) == :ok
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end
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@tag task_id: 1
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test "calculate! returns what the operation does: an ok string" do
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assert RPNCalculator.calculate!([], fn _ -> "ok" end) == "ok"
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end
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@tag task_id: 1
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test "let it crash" do
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assert_raise(RuntimeError, fn ->
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RPNCalculator.calculate!([], fn _ -> raise "test error" end)
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end)
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end
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@tag task_id: 2
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test "calculate returns the result of the operation (a string) wrapped in an :ok tuple" do
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assert RPNCalculator.calculate([], fn _ -> "operation completed" end) ==
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{:ok, "operation completed"}
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end
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@tag task_id: 2
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test "calculate returns the result of the operation (an atom) wrapped in an :ok tuple" do
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assert RPNCalculator.calculate([], fn _ -> :success end) ==
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{:ok, :success}
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end
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@tag task_id: 2
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test "rescue the crash, no message" do
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assert RPNCalculator.calculate([], fn _ -> raise "test error" end) == :error
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end
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@tag task_id: 3
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test "calculate_verbose returns the result of the operation (a string) wrapped in an :ok tuple" do
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assert RPNCalculator.calculate_verbose([], fn _ -> "operation completed" end) ==
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{:ok, "operation completed"}
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end
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@tag task_id: 3
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test "calculate_verbose returns the result of the operation (an atom) wrapped in an :ok tuple" do
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assert RPNCalculator.calculate_verbose([], fn _ -> :success end) ==
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{:ok, :success}
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end
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@tag task_id: 3
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test "rescue the crash, get error tuple with message" do
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assert RPNCalculator.calculate_verbose([], fn _ -> raise ArgumentError, "test error" end) ==
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{:error, "test error"}
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end
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end
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ExUnit.start()
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ExUnit.configure(exclude: :pending, trace: true, seed: 0)
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