diff --git a/Cargo.lock b/Cargo.lock
index 58c81c4..903555b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -99,6 +99,12 @@ dependencies = [
  "windows-sys 0.45.0",
 ]
 
+[[package]]
+name = "diff"
+version = "0.1.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "56254986775e3233ffa9c4d7d3faaf6d36a2c09d30b20687e9f88bc8bafc16c8"
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -206,6 +212,16 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "pretty_assertions"
+version = "1.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3ae130e2f271fbc2ac3a40fb1d07180839cdbbe443c7a27e1e3c13c5cac0116d"
+dependencies = [
+ "diff",
+ "yansi",
+]
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.104"
@@ -232,6 +248,7 @@ dependencies = [
  "ariadne",
  "chumsky",
  "logos",
+ "pretty_assertions",
 ]
 
 [[package]]
diff --git a/Cargo.toml b/Cargo.toml
index 8d6ae4e..1f26477 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,3 +7,6 @@ edition = "2024"
 ariadne = { version = "0.6.0", features = ["auto-color"]}
 chumsky = { version = "0.12.0", features = ["pratt"] }
 logos = "0.16.0"
+
+[dev-dependencies]
+pretty_assertions = "1.4.1"
diff --git a/README.md b/README.md
index a763a9f..604ce3f 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@ Values:
 `double x = x * 2`
 ```rust
 double = fn([x] [x]) (
-    ^x = x * 2;
+    ^x := x * 2;
 );
 ```
 
@@ -22,7 +22,7 @@ double = fn([x] [x]) (
 ```rust
 ;; same as apply = fn([f x] [x]) ( ... )
 fn apply([f x] [x]) (
-    ^x = f x;
+    ^x := f x;
 );
 
 ;; f would have to be like fn([a] [a]) ( ... )
@@ -37,10 +37,10 @@ fn apply([f x] [x]) (
 ```
 ```rust
 fn make_counter([] [inc]) (
-    i = 0;
-    ^inc = fn([] [i]) (
-        i = i + 1;
-        ^i = i;
+    i := 0;
+    ^inc := fn([] [i]) (
+        i := i + 1;
+        ^i := i;
     );
 
     ;; or fn ^inc(...)
@@ -51,20 +51,20 @@ fn make_counter([] [inc]) (
 ```rust
 ;; omitting the return means it's the same
 fn desat([h s l a]) (
-    ^s = ^s / 2;
+    ^s := ^s / 2;
 )
 ```
 
 `compose f g x = f (g x)`
 ```rust
 fn compose([f g x] [x]) (
-    ^x = g x;
-    ^x = f x;
+    ^x := g x;
+    ^x := f x;
 )
 
-f = fn([x]) (^x = x + 2;);
-g = fn([x]) (^x = x * 3;);
-f2 = f;
+f := fn([x]) (^x := x + 2;);
+g := fn([x]) (^x := x * 3;);
+f2 := f;
 
 compose f g x == (((compose f) g) x) == compose {f: f2, g} x == compose {x} f g
 ```
diff --git a/src/ast.rs b/src/ast.rs
index 5d300eb..24a65de 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -85,13 +85,17 @@ impl fmt::Display for Ast {
                 body,
             } => {
                 let body: Vec<_> = body.iter().map(|stmt| format!("{stmt}")).collect();
-                write!(
-                    f,
-                    "(fn ([{}] [{}]) {})",
-                    inputs.join(" "),
-                    outputs.join(" "),
-                    body.join(" ")
-                )
+                if body.is_empty() {
+                    write!(f, "(fn ([{}] [{}]))", inputs.join(" "), outputs.join(" "),)
+                } else {
+                    write!(
+                        f,
+                        "(fn ([{}] [{}]) {})",
+                        inputs.join(" "),
+                        outputs.join(" "),
+                        body.join(" ")
+                    )
+                }
             }
             Ast::Map(body) => {
                 let body: Vec<_> = body.iter().map(|stmt| format!("{stmt}")).collect();
diff --git a/src/tests/parser.rs b/src/tests/parser.rs
index 67714e3..723e858 100644
--- a/src/tests/parser.rs
+++ b/src/tests/parser.rs
@@ -1,7 +1,7 @@
 use crate::parser::*;
 
 fn test(src: &str, expected: &str) {
-    assert_eq!(
+    pretty_assertions::assert_eq!(
         parse(src)
             .into_iter()
             .map(|stmt| format!("{stmt}"))
@@ -16,10 +16,27 @@ fn double() {
     test(
         "
     fn double([x]) (
-        ^x = x * 2;
+        ^x := x * 2;
     )
 ",
-        "(set! double (fn ([x] [x]) (^x = (x * 2))))",
+        "
+(set! double (fn ([x] [x]) (set! ^x (x * 2))))
+",
+    );
+}
+
+#[test]
+fn apply() {
+    test(
+        "
+# same as apply = fn([f x] [x]) ( ... )
+fn apply([f x] [x]) (
+    ^x := f x;
+)
+
+# f would have to be like fn([a] [a]) ( ... )
+        ",
+        "(set! apply (fn ([f x] [x]) (set! ^x (f x))))",
     );
 }
 
@@ -28,15 +45,92 @@ fn make_counter() {
     test(
         "
 fn make_counter([] [inc]) (
-    i = 0;
-    ^inc = fn([] [i]) (
-        i = i + 1;
-        ^i = i;
+    i := 0;
+    ^inc := fn([] [i]) (
+        i := i + 1;
+        ^i := i;
     );
 
     # or fn ^inc(...)
 )
 ",
-        "(set! make_counter (fn ([] [inc]) (i = 0) (^inc = (fn ([] [i]) (i = (i + 1)) (^i = i)))))",
+        "
+(set! make_counter \
+  (fn ([] [inc]) \
+    (set! i 0) \
+    (set! ^inc (fn ([] [i]) \
+      (set! i (i + 1)) \
+      (set! ^i i)))))
+",
+    );
+}
+
+#[test]
+fn desat() {
+    test(
+        "
+fn desat([h s l a]) (
+    ^s := ^s / 2;
+)
+",
+        "(set! desat (fn ([h s l a] [h s l a]) (set! ^s (^s / 2))))",
+    );
+}
+
+#[test]
+fn compose() {
+    test(
+        "
+fn compose([f g x] [x]) (
+    ^x := g x;
+    ^x := f x;
+);
+
+f := fn([x]) (^x := x + 2;);
+g := fn([x]) (^x := x * 3;);
+f2 := f;
+
+compose f g x = (((compose f) g) x);
+compose f g x = compose {^f := f2; ^g} x;
+compose f g x = compose {^x} f g;
+",
+        "
+(set! compose (fn ([f g x] [x]) (set! ^x (g x)) (set! ^x (f x))));
+(set! f (fn ([x] [x]) (set! ^x (x + 2))));
+(set! g (fn ([x] [x]) (set! ^x (x * 3))));
+(set! f2 f);
+((((compose f) g) x) = ((((((compose f)) g)) x)));
+((((compose f) g) x) = ((compose {(set! ^f f2) ^g}) x));
+((((compose f) g) x) = (((compose {^x}) f) g))
+        ",
+    );
+}
+
+#[test]
+fn id() {
+    test("fn id([x]) ()", "(set! id (fn ([x] [x])))");
+}
+
+#[test]
+fn map() {
+    test(
+        "
+foo := 3;
+{
+    x := 42;
+    ^twice_x := x * 2;
+    ^thrice_x := x * 3;
+    ^foo;
+}
+",
+        "
+(set! foo 3);
+{\
+  (set! x 42) \
+  (set! ^twice_x (x * 2)) \
+  (set! ^thrice_x (x * 3)) \
+  ^foo\
+}
+",
     );
 }