add introduction to unit type

moonbitlang · Apr 3, 2024 · 6a35a7d · 6a35a7d
1 parent 9956335
commit 6a35a7d
Show file tree

Hide file tree

Showing 2 changed files with 18 additions and 4 deletions.
diff --git a/course2/course_en.md b/course2/course_en.md
@@ -241,6 +241,15 @@ MoonBit has a static type system: the compiler performs type checking before run
 
 ---
 
+# Unit
+
+- A type which has only one value.
+  - `(): Unit`
+- It enables the treatment of statements as expressions.
+  - In MoonBit, all statements evaluate to `()`.
+
+---
+
 # Other Data Types
 
 - MoonBit has a rich type system.
@@ -530,8 +539,9 @@ $$\begin{align}
     \mapsto & \texttt{10}
 \end{align}$$
 
-A conditional expression must have two branches.
-- Otherwise, if the condition is false, what should be the value of the expression?
+If the true branch ends with a statement, the false branch can be omitted.
+- Implicitly, there is a hidden false branch returning `()`.
+- Therefore, the two branches still have the same type, i.e., `Unit`.
 
 ---
 

diff --git a/course2/lecture_en.md b/course2/lecture_en.md
@@ -181,9 +181,13 @@ We can access the data by using zero-based indexing.
 - `(2023, 10, 24).0 == 2023`
 - `(2023, 10, 24).1 == 10`
 
+#### Unit
+
+In MoonBit, the `Unit` type represents a singular value denoted as `()`. Though seemingly useless, it holds significant implications as it enables the treatment of statements as expressions: in MoonBit, all statements evaluate to `()`.
+
 #### Other Data Types
 
-MoonBit has a rich type system, which includes many other types that we have not yet discussed, such as function types, unit types, and list types. These types will be explored in detail in future modules.
+MoonBit has a rich type system, which includes many other types that we have not yet discussed, such as function types and list types. These types will be explored in detail in future modules.
 
 ### Expression Evaluation
 
@@ -363,7 +367,7 @@ $$\begin{align}
     \mapsto & \texttt{10}
 \end{align}$$
 
-A conditional expression must have two branches. Otherwise, if the condition is false, what should be the value of the expression?
+If the true branch ends with a statement, the false branch can be omitted. Implicitly, there is a hidden false branch returning `()`, and therefore the two branches still have the same type, i.e., `Unit`.
 
 ## Summary