Closures and function definitions 🚀

Supports:
- Closure syntax: `(x, y) => z`
- Shorthand for a single argument: `x => y`
- Function syntax: `let f(x) = y`
- Capturing of variables from the environment
- Error messages for too few / many passed arguments

Does not support:
- Named arguments
- Variadic arguments with `..`

3 files changed, 24 insertions, 8 deletions

diff --git a/tests/typ/control/for.typ b/tests/typ/control/for.typ
index 36bce447..5eaa6ae8 100644
--- a/tests/typ/control/for.typ
+++ b/tests/typ/control/for.typ
@@ -57,9 +57,6 @@
 #test(type(for v in () []), "template")
 
 ---
-// Error: 14-19 unknown variable
-#let error = error
-
 // Uniterable expression.
 // Error: 11-15 cannot loop over boolean
 #for v in true {}
diff --git a/tests/typ/control/let.typ b/tests/typ/control/let.typ
index 8df29b11..7752ec90 100644
--- a/tests/typ/control/let.typ
+++ b/tests/typ/control/let.typ
@@ -7,9 +7,31 @@
 #let x
 #test(x, none)
 
+// Error: 9 expected expression
+#let y =
+#test(y, none)
+
 // Manually initialized with one.
-#let x = 1
-#test(x, 1)
+#let z = 1
+#test(z, 1)
+
+---
+// Syntax sugar for function definitions.
+#let background = #239dad
+#let box(body) = box(width: 2cm, height: 1cm, color: background, body)
+#box[Hi!]
+
+// Error: 13 expected body
+#let func(x)
+
+// Error: 2-6 unknown variable
+{func}
+
+// Error: 15 expected expression
+#let func(x) =
+
+// Error: 2-6 unknown variable
+{func}
 
 ---
 // Termination.
diff --git a/tests/typ/control/while.typ b/tests/typ/control/while.typ
index 7ad70372..acf7951e 100644
--- a/tests/typ/control/while.typ
+++ b/tests/typ/control/while.typ
@@ -26,9 +26,6 @@
 #test(type(while false []), "template")
 
 ---
-// Error: 14-19 unknown variable
-#let error = error
-
 // Condition must be boolean.
 // Error: 8-14 expected boolean, found template
 #while [nope] [nope]