GADT section done

Chapter04/Cargo.toml

@@ -11,3 +11,7 @@ path = "generics.rs"
 [[bin]]
 name = "polymorphism"
 path = "polymorphism.rs"
+
+[[bin]]
+name = "gadt"
+path = "gadt.rs"

Chapter04/gadt.rs

@@ -0,0 +1,84 @@
+
+type JSJIT = u64;
+
+enum JSJITorExpr {
+   Jit { label: JSJIT },
+   Expr { expr: Box<JSExpr> }
+}
+
+enum JSExpr {
+   Integer { value: u64 },
+   String { value: String },
+   OperatorAdd { lexpr: Box<JSJITorExpr>, rexpr: Box<JSJITorExpr> },
+   OperatorMul { lexpr: Box<JSJITorExpr>, rexpr: Box<JSJITorExpr> }
+}
+
+fn jump(l: JSJIT) -> JSJITorExpr
+{
+   //jump to compiled code
+   //this depends on implementation
+   //so we will just leave this as a stub
+   JSJITorExpr::Jit { label: 0 }
+}
+fn eval(e: JSJITorExpr) -> JSJITorExpr
+{
+   match e
+   {
+      JSJITorExpr::Jit { label: label } => jump(label),
+      JSJITorExpr::Expr { expr: expr } => {
+         let rawexpr = *expr;
+         match rawexpr
+         {
+            JSExpr::Integer {..} => JSJITorExpr::Expr { expr: Box::new(rawexpr) },
+            JSExpr::String {..} => JSJITorExpr::Expr { expr: Box::new(rawexpr) },
+            JSExpr::OperatorAdd { lexpr: l, rexpr: r } => {
+               let l = eval(*l);
+               let r = eval(*r);
+               //call add op codes for possible l,r representations
+               //should return wrapped value from above
+               JSJITorExpr::Jit { label: 0 }
+            }
+            JSExpr::OperatorMul { lexpr: l, rexpr: r } => {
+               let l = eval(*l);
+               let r = eval(*r);
+               //call mul op codes for possible l,r representations
+               //should return wrapped value from above
+               JSJITorExpr::Jit { label: 0 }
+            }
+         }
+      }
+   }
+}
+
+pub trait HList: Sized {}
+
+pub struct HNil;
+impl HList for HNil {}
+
+pub struct HCons<H, T> {
+    pub head: H,
+    pub tail: T,
+}
+impl<H, T: HList> HList for HCons<H, T> {}
+impl<H, T> HCons<H, T> {
+    pub fn pop(self) -> (H, T) {
+        (self.head, self.tail)
+    }
+}
+
+fn main()
+{
+   let hl = HCons {
+      head: 2,
+      tail: HCons {
+         head: "abcd".to_string(),
+         tail: HNil
+      }
+   };
+   let (h1,t1) = hl.pop();
+   let (h2,t2) = t1.pop();
+   //this would fail
+   //HNil has no .pop method
+   //t2.pop();
+
+}