TylerPachal · September 26, 2023 15:14 · Sep 26, 2023 · Sep 18, 2023 · Sep 18, 2023 · Sep 18, 2023
diff --git a/custom-compile-time-warnings-in-elixir.md b/custom-compile-time-warnings-in-elixir.md
@@ -104,7 +104,7 @@ defmodule Country do
 end
 ```
 
-Now when we compile the code, we'll get a more helpful preceding the warning from the Elixir compiler:
+Now when we compile the code, we'll get a more helpful warning preceding the warning from the Elixir compiler:
 
 ```bash
 $ mix compile --force

diff --git a/custom-compile-time-warnings-in-elixir.md b/custom-compile-time-warnings-in-elixir.md
@@ -9,7 +9,7 @@ When you are writing macros which run at compile-time, it can be useful to emit
 
 Say you are using a text file to dynamically define function clauses.  Our function will be called `official_language/1`; its only argument is the name of a country, and it returns an `:ok` tuple with a list of the countries official languages.  
 
-Foruntaley, for our implementation we have been provided with a `official_languages.csv` file that contains the mappings we need to perform this lookup.
+Fortunately, for our implementation we have been provided with a `official_languages.csv` file that contains the mappings we need to perform this lookup:
 
 ```text
 Belgium,"Dutch,French,German"
@@ -19,7 +19,7 @@ Mexico,Spanish
 Italy,Italian
 ```
 
-Implementing this with metaprogamming would look like this:
+Using the contents of this file to dynamically define our function with metaprogamming would look like this:
 
 ```elixir
 defmodule Country do
@@ -74,7 +74,7 @@ warning: this clause cannot match because a previous clause at line 8 always mat
 
 ### The Solution
 
-It would be better if we emitted a more helpful error message (after idenitfying non-unique country keys).  This can be done using [`IO.warn/1`](https://hexdocs.pm/elixir/1.12.3/IO.html#warn/1).
+It would be better if we emitted a more helpful error message (after identifying non-unique country keys).  This can be done using [`IO.warn/1`](https://hexdocs.pm/elixir/1.12.3/IO.html#warn/1):
 
 ```elixir
 defmodule Country do
@@ -104,7 +104,7 @@ defmodule Country do
 end
 ```
 
-Now when we compile the code, we'll get a more helpful preceeding the warning from the Elixir compiler:
+Now when we compile the code, we'll get a more helpful preceding the warning from the Elixir compiler:
 
 ```bash
 $ mix compile --force

diff --git a/custom-compile-time-warnings-in-elixir.md b/custom-compile-time-warnings-in-elixir.md
@@ -7,7 +7,17 @@ When you are writing macros which run at compile-time, it can be useful to emit
 
 ### Example Scenario
 
-Say you are using a text file to dynamically function clauses.  Our function is called `official_language/1` and it takes the name of a country, and returns a list of its official languages (some countries have more than one!).  Foruntaley, we have been provided with a `official_languages.csv` file that contains the mappings we need to perform this lookup.
+Say you are using a text file to dynamically define function clauses.  Our function will be called `official_language/1`; its only argument is the name of a country, and it returns an `:ok` tuple with a list of the countries official languages.  
+
+Foruntaley, for our implementation we have been provided with a `official_languages.csv` file that contains the mappings we need to perform this lookup.
+
+```text
+Belgium,"Dutch,French,German"
+Brazil,Portugese
+Canada,"English,French"
+Mexico,Spanish
+Italy,Italian
+```
 
 Implementing this with metaprogamming would look like this:
 
@@ -32,17 +42,11 @@ end
 
 _(Read more about `@external_resource` [here](https://hexdocs.pm/elixir/1.12.3/Module.html#module-external_resource))_
 
-If you have the following `official_languages.csv` file in your `priv` repo, you'd be able to do the following function calls:
-
-```text
-Belgium,"Dutch,French,German"
-Brazil,Portugese
-Canada,"English,French"
-Mexico,Spanish
-Italy,Italian
-```
+With the CSV file in place, you'd be able to make the following function calls:
 
 ```bash
+$ iex -S mix
+
 iex(1)> Country.official_language("Canada")   
 {:ok, ["English", "French"]}
 
@@ -54,7 +58,7 @@ iex(2)> Country.official_language("Tylerland")
 
 ### The Problem
 
-What if there was accidentally a duplicate country in your CSV file?  Maybe the file contained an additional (and incorrect) line of `Brazil,Spanish`.
+What if there was accidentally a duplicate country in your CSV file?  Maybe the file contained an additional (and incorrect) line: `Brazil,Spanish`.
 
 We'd see a compiler warning, which is not helpful at narrowing down which country was the duplicate:
 
@@ -100,7 +104,7 @@ defmodule Country do
 end
 ```
 
-Now when we compile the code, we'll get a more helpful warning (along with the original warning)
+Now when we compile the code, we'll get a more helpful preceeding the warning from the Elixir compiler:
 
 ```bash
 $ mix compile --force

diff --git a/custom-compile-time-warnings-in-elixir.md b/custom-compile-time-warnings-in-elixir.md
@@ -1,9 +1,9 @@
-Custom Compile-Time warnings in Elixir
+Custom Compile-Time Warnings in Elixir Using IO.warn/1
 ---
 
 When you are writing macros which run at compile-time, it can be useful to emit custom compile-time warnings.
 
-[Skip to the answer](#the-solution)
+[Skip down to The Solution](#the-solution)
 
 ### Example Scenario
 
@@ -70,7 +70,7 @@ warning: this clause cannot match because a previous clause at line 8 always mat
 
 ### The Solution
 
-It would be better if we emitted a more helpful error message (after idenitfying non-unique country keys).  This can be done using `IO.warn/1`.
+It would be better if we emitted a more helpful error message (after idenitfying non-unique country keys).  This can be done using [`IO.warn/1`](https://hexdocs.pm/elixir/1.12.3/IO.html#warn/1).
 
 ```elixir
 defmodule Country do

diff --git a/custom-compile-time-warnings-in-elixir.md b/custom-compile-time-warnings-in-elixir.md
@@ -0,0 +1,120 @@
+Custom Compile-Time warnings in Elixir
+---
+
+When you are writing macros which run at compile-time, it can be useful to emit custom compile-time warnings.
+
+[Skip to the answer](#the-solution)
+
+### Example Scenario
+
+Say you are using a text file to dynamically function clauses.  Our function is called `official_language/1` and it takes the name of a country, and returns a list of its official languages (some countries have more than one!).  Foruntaley, we have been provided with a `official_languages.csv` file that contains the mappings we need to perform this lookup.
+
+Implementing this with metaprogamming would look like this:
+
+```elixir
+defmodule Country do
+  @external_resource "priv/official_languages.csv"
+  @contents File.read!("priv/official_languages.csv")
+
+  rows = NimbleCSV.RFC4180.parse_string(@contents)
+
+  for [country, languages] <- rows do
+    languages_list = String.split(languages, ",")
+
+    def official_language(unquote(country)) do
+      {:ok, unquote(languages_list)}
+    end
+  end
+
+  def official_language(_country), do: {:error, :unknown_country}
+end
+```
+
+_(Read more about `@external_resource` [here](https://hexdocs.pm/elixir/1.12.3/Module.html#module-external_resource))_
+
+If you have the following `official_languages.csv` file in your `priv` repo, you'd be able to do the following function calls:
+
+```text
+Belgium,"Dutch,French,German"
+Brazil,Portugese
+Canada,"English,French"
+Mexico,Spanish
+Italy,Italian
+```
+
+```bash
+iex(1)> Country.official_language("Canada")   
+{:ok, ["English", "French"]}
+
+iex(2)> Country.official_language("Tylerland")
+{:error, :unknown_country}
+```
+
+---
+
+### The Problem
+
+What if there was accidentally a duplicate country in your CSV file?  Maybe the file contained an additional (and incorrect) line of `Brazil,Spanish`.
+
+We'd see a compiler warning, which is not helpful at narrowing down which country was the duplicate:
+
+```bash
+$ mix compile --force
+
+Compiling 1 file (.ex)
+warning: this clause cannot match because a previous clause at line 8 always matches
+  lib/country.ex:10
+```
+
+---
+
+### The Solution
+
+It would be better if we emitted a more helpful error message (after idenitfying non-unique country keys).  This can be done using `IO.warn/1`.
+
+```elixir
+defmodule Country do
+  @external_resource "priv/official_languages.csv"
+  @contents File.read!("priv/official_languages.csv")
+
+  rows = NimbleCSV.RFC4180.parse_string(@contents)
+
+  # New part here!
+  countries = Enum.map(rows, fn [country, _] -> country end)
+  non_unique_countries = countries -- Enum.uniq(countries)
+  if non_unique_countries != [] do
+    IO.warn("""
+    Non-unique country in #{@external_resource}: #{inspect(non_unique_countries)}
+    """)
+  end
+
+  for [country, languages] <- rows do
+    languages_list = String.split(languages, ",")
+
+    def official_language(unquote(country)) do
+      {:ok, unquote(languages_list)}
+    end
+  end
+
+  def official_language(_country), do: {:error, :unknown_country}
+end
+```
+
+Now when we compile the code, we'll get a more helpful warning (along with the original warning)
+
+```bash
+$ mix compile --force
+
+Compiling 1 file (.ex)
+warning: Non-unique country in priv/official_languages.csv: ["Brazil"]
+
+  lib/playground.ex:11: (module)
+  (elixir 1.13.3) src/elixir_compiler.erl:73: :elixir_compiler.dispatch/4
+  (elixir 1.13.3) src/elixir_compiler.erl:58: :elixir_compiler.compile/3
+  (elixir 1.13.3) src/elixir_module.erl:369: :elixir_module.eval_form/6
+  (elixir 1.13.3) src/elixir_module.erl:105: :elixir_module.compile/5
+  (elixir 1.13.3) src/elixir_lexical.erl:15: :elixir_lexical.run/3
+
+warning: this clause cannot match because a previous clause at line 19 always matches
+  lib/playground.ex:19
+```
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