From 4efb34bd728732101432843ed0bfbeb971272287 Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Thu, 21 Dec 2023 14:03:41 +0000
Subject: Add chapters

---
 .../lesson04_generic_functions/code.gleam          | 19 ++++++++++++++++
 .../lesson04_generic_functions/text.html           | 25 ++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 src/content/chapter1_functions/lesson04_generic_functions/code.gleam
 create mode 100644 src/content/chapter1_functions/lesson04_generic_functions/text.html

(limited to 'src/content/chapter1_functions/lesson04_generic_functions')

diff --git a/src/content/chapter1_functions/lesson04_generic_functions/code.gleam b/src/content/chapter1_functions/lesson04_generic_functions/code.gleam
new file mode 100644
index 0000000..e232bf8
--- /dev/null
+++ b/src/content/chapter1_functions/lesson04_generic_functions/code.gleam
@@ -0,0 +1,19 @@
+import gleam/io
+
+pub fn main() {
+  let add_one = fn(x) { x + 1 }
+  let exclaim = fn(x) { x <> "!" }
+
+  // Invalid, Int and String are not the same type
+  // twice(10, exclaim)
+
+  // Here the type variable is replaced by the type Int
+  io.debug(twice(10, add_one))
+
+  // Here the type variable is replaced by the type String
+  io.debug(twice("Hello", exclaim))
+}
+
+fn twice(argument: value, function: fn(value) -> value) -> value {
+  function(function(argument))
+}
diff --git a/src/content/chapter1_functions/lesson04_generic_functions/text.html b/src/content/chapter1_functions/lesson04_generic_functions/text.html
new file mode 100644
index 0000000..1369c93
--- /dev/null
+++ b/src/content/chapter1_functions/lesson04_generic_functions/text.html
@@ -0,0 +1,25 @@
+<p>
+  Up until now each function has accepted precisely one type for each of its
+  arguments.
+</p>
+<p>
+  The <code>twice</code> function for example only worked with functions that
+  would take and return ints. This is overly restrictive, it should be possible
+  to use this function with any type, so long as the function and the initial
+  value are compatible.
+</p>
+<p>
+  To enable this Gleam support <em>generics</em>, also known as <em>parametric
+  polymorphism</em>.
+</p>
+<p>
+  This works by instead of specifying a concrete type, a type variable is used
+  which stands in for whatever specific type is being used when the function is
+  called. These type variable are written with a lowercase name.
+</p>
+<p>
+  Type variables are not like an <code>any</code> type, they get replaced with a
+  specific type each time the function is called. Try uncommenting
+  <code>twice(10, exclaim)</code> to see the compiler error from trying to use a
+  type variable as an int and a string at the same time.
+</p>
-- 
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