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diff --git a/src/content/chapter0_basics/lesson12_type_aliases/code.gleam b/src/content/chapter0_basics/lesson12_type_aliases/code.gleam
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+++ b/src/content/chapter0_basics/lesson12_type_aliases/code.gleam
@@ -0,0 +1,12 @@
+import gleam/io
+
+pub type UserId =
+  Int
+
+pub fn main() {
+  let one: UserId = 1
+  let two: Int = 2
+
+  // UserId and Int are the same type
+  io.debug(one == two)
+}
diff --git a/src/content/chapter0_basics/lesson12_type_aliases/text.html b/src/content/chapter0_basics/lesson12_type_aliases/text.html
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+<p>
+  A type alias can be used to refer to a type by a different name. Giving a type
+  an alias doesn't make a new type, it is still the same type.
+</p>
+<p>
+  When the <code>pub</code> keyword is used the type alias is public and can be
+  referred to by other module.
+</p>
diff --git a/src/content/chapter0_basics/lesson12_blocks/code.gleam b/src/content/chapter0_basics/lesson13_blocks/code.gleam
index 31e4729..31e4729 100644
--- a/src/content/chapter0_basics/lesson12_blocks/code.gleam
+++ b/src/content/chapter0_basics/lesson13_blocks/code.gleam
diff --git a/src/content/chapter0_basics/lesson12_blocks/text.html b/src/content/chapter0_basics/lesson13_blocks/text.html
index bc82e39..bc82e39 100644
--- a/src/content/chapter0_basics/lesson12_blocks/text.html
+++ b/src/content/chapter0_basics/lesson13_blocks/text.html
diff --git a/src/content/chapter0_basics/lesson13_lists/code.gleam b/src/content/chapter0_basics/lesson14_lists/code.gleam
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--- a/src/content/chapter0_basics/lesson13_lists/code.gleam
+++ b/src/content/chapter0_basics/lesson14_lists/code.gleam
diff --git a/src/content/chapter0_basics/lesson13_lists/text.html b/src/content/chapter0_basics/lesson14_lists/text.html
index dd07cd7..dd07cd7 100644
--- a/src/content/chapter0_basics/lesson13_lists/text.html
+++ b/src/content/chapter0_basics/lesson14_lists/text.html
diff --git a/src/content/chapter0_basics/lesson15_constants/code.gleam b/src/content/chapter0_basics/lesson15_constants/code.gleam
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+++ b/src/content/chapter0_basics/lesson15_constants/code.gleam
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+import gleam/io
+
+const ints: List(Int) = [1, 2, 3]
+
+const floats = [1.0, 2.0, 3.0]
+
+pub fn main() {
+  io.debug(ints)
+  io.debug(ints == [1, 2, 3])
+
+  io.debug(floats)
+  io.debug(floats == [1.0, 2.0, 3.0])
+}
diff --git a/src/content/chapter0_basics/lesson15_constants/text.html b/src/content/chapter0_basics/lesson15_constants/text.html
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+<p>
+  As well as let assignments Gleam also has constants, which are defined at the
+  top level of a module.
+</p>
+<p>
+  Constants must be literal values, functions cannot be used in their
+  definitions.
+</p>
+<p>
+  Constants may be useful for values that are used throughout your program,
+  permitting them to be named and to ensure there are no differences in the
+  definition between each use.
+</p>
+<p>
+  Using a constant may be more efficient than creating the same value in
+  multiple functions, though the exact performance characteristics will depend
+  on the runtime and whether compiling to Erlang or JavaScript.
+</p>