Add Optional type for filter pipes

Introduces `StaticType::Optional` to represent values that can be either
of a specific type `T` or `Void`. This is crucial for handling
situations in filter pipes where an expression might not always produce
a value.

The type checker now correctly deduces and propagates this `Optional`
type, and the pipeline operator (`pipe`) is updated to unwrap
`Optional(T)` results, yielding `T`. This ensures that the type system
accurately reflects the potential absence of values in intermediate
pipeline steps.

Also includes a minor rename in the tuple-struct example from `pipe` to
`p` for clarity, and registers the `streams` module.

examples/tuple-struct.myc

@@ -1,7 +1,7 @@
 ;; Benchmark: 890ns
 ;; Benchmark-Repeat: 2258
 (do
-     (def pipe (fn [conf]
+     (def p (fn [conf]
        (do
           (def [str s] conf)
           (def [f ss] s)
@@ -10,5 +10,5 @@
       )
      )
 
-     (pipe ["btc" [:close "cls"]])
+     (p ["btc" [:close "cls"]])
    )

src/ast/compiler/type_checker.rs

@@ -373,8 +373,8 @@ impl TypeChecker {
                     }
                     else_typed = Some(Box::new(et));
                 } else {
-                    // If without else returns Void or Optional(Then)
-                    final_ty = StaticType::Any; // Delphi: MakeOptional(Then)
+                    // If without else returns Optional(Then) (T | Void)
+                    final_ty = StaticType::Optional(Box::new(then_typed.ty.clone()));
                 }
 
                 (
@@ -394,7 +394,12 @@ impl TypeChecker {
                 }
                 let typed_lambda = self.check_node(*lambda, ctx)?;
                 let ret_ty = if let StaticType::Function(sig) = &typed_lambda.ty {
-                    sig.ret.clone()
+                    // If the lambda returns an Optional(T), the pipeline filters Void and stores T!
+                    if let StaticType::Optional(inner) = &sig.ret {
+                        *inner.clone()
+                    } else {
+                        sig.ret.clone()
+                    }
                 } else {
                     StaticType::Any
                 };

src/ast/rtl/mod.rs

@@ -13,4 +13,5 @@ pub fn register(env: &Environment) {
     datetime::register(env);
     math::register(env);
     series::register(env);
+    streams::register(env);
 }

src/ast/types.rs

@@ -295,6 +295,7 @@ pub enum StaticType {
     DateTime,
     Text,
     Keyword,
+    Optional(Box<StaticType>),           // Represents T | Void (e.g. for filter pipes)
     List(Box<StaticType>),               // Legacy / Dynamic list
     Series(Box<StaticType>),             // Time series of a specific type
     Tuple(Vec<StaticType>),              // Heterogeneous fixed-size
@@ -318,7 +319,8 @@ impl fmt::Display for StaticType {
             StaticType::DateTime => write!(f, "datetime"),
             StaticType::Text => write!(f, "text"),
             StaticType::Keyword => write!(f, "keyword"),
-            StaticType::List(inner) => write!(f, "[{}]", inner),
+            StaticType::Optional(inner) => write!(f, "optional({})", inner),
+            StaticType::List(inner) => write!(f, "list({})", inner),
             StaticType::Series(inner) => write!(f, "series<{}>", inner),
             StaticType::Tuple(elements) => {
                 write!(f, "[")?;
@@ -371,6 +373,10 @@ impl StaticType {
         }
 
         match (self, other) {
+            // Optional(T) is assignable from T or Void
+            (StaticType::Optional(inner), other) => {
+                matches!(other, StaticType::Void) || inner.is_assignable_from(other)
+            }
             // A Vector is a Tuple
             (StaticType::Tuple(elements), StaticType::Vector(inner, len)) => {
                 if elements.len() != *len {

src/integration_test.rs

@@ -360,6 +360,38 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_pipeline_optional_type() {
+        let env = Environment::new();
+        // The lambda uses an `if` without an `else` returning a float constant.
+        // The TypeChecker should deduce `Optional(Float)` for the lambda body,
+        // and correctly unwrap it to `Series(Float)` for the pipeline output.
+        let source = "(do
+            (def src (create-random-ohlc 42 10))
+            (def filtered 
+                (pipe [src] 
+                    (fn [tick] 
+                        (if true 
+                            42.0
+                        )
+                    )
+                )
+            )
+            filtered
+        )";
+        let res = env.run_script(source);
+        if let Err(e) = &res {
+            panic!("Script failed to compile/run: {:?}", e);
+        }
+        
+        let val = res.unwrap();
+        if let crate::ast::types::Value::Object(obj) = val {
+            assert_eq!(obj.type_name(), "PipelineNode");
+        } else {
+            panic!("Expected an Object(PipelineNode)");
+        }
+    }
+
     #[test]
     fn test_multi_level_destructuring() {
         let env = Environment::new();