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 `..`

1 files changed, 66 insertions, 0 deletions

diff --git a/tests/typ/expr/closure.typ b/tests/typ/expr/closure.typ
new file mode 100644
index 00000000..d05acaa4
--- /dev/null
+++ b/tests/typ/expr/closure.typ
@@ -0,0 +1,66 @@
+// Test closures.
+
+---
+// Ref: false
+
+// Basic closure without captures.
+{
+    let adder = (x, y) => x + y
+    test(adder(2, 3), 5)
+}
+
+// Pass closure as argument and return closure.
+// Also uses shorthand syntax for a single argument.
+{
+    let chain = (f, g) => (x) => f(g(x))
+    let f = x => x + 1
+    let g = x => 2 * x
+    let h = chain(f, g)
+    test(h(2), 5)
+}
+
+// Capture environment.
+{
+    let mark = "?"
+    let greet = {
+        let hi = "Hi"
+        name => {
+            hi + ", " + name + mark
+        }
+    }
+
+    test(greet("Typst"), "Hi, Typst?")
+
+    mark = "!"
+    test(greet("Typst"), "Hi, Typst!")
+}
+
+// Don't leak environment.
+{
+    // Error: 18-19 unknown variable
+    let func() = x
+    let x = "hi"
+
+    test(func(), error)
+}
+
+---
+// Ref: false
+
+// Too few arguments.
+{
+    let types(x, y) = "[" + type(x) + ", " + type(y) + "]"
+    test(types(14%, 12pt), "[relative, length]")
+
+    // Error: 16-22 missing argument: y
+    test(types("nope"), "[string, none]")
+}
+
+// Too many arguments.
+{
+    let f(x) = x + 1
+
+    // Error: 2:10-2:15 unexpected argument
+    // Error: 1:17-1:24 unexpected argument
+    f(1, "two", () => x)
+}