feat: dependent types — refinement types, type aliases, type annotations

- Lexer: added 'type' and 'where' keywords
- AST: TypeExpr::Refined, Declaration::TypeAlias, LetDecl.type_annotation
- Parser: parse_type_alias_decl, parse_type_expr (Named, Generic, where)
- Type system: Type::Refined, Predicate/PredicateExpr with evaluate_static()
- Errors: RefinementViolation, TypeAliasCycle (Elm-style messages)
- Checker: type alias registry, resolve_type_expr, ast_to_predicate,
  static refinement checking for literal values
- Codegen: Phase 1b runtime guards, predicate_to_js helper

Syntax: type PositiveInt = Int where value > 0
        let count: PositiveInt = 5  -- static check passes
        let count: PositiveInt = -1 -- compile error

118 tests pass (16 in ds-types, 5 new for refinements)

compiler/ds-codegen/src/js_emitter.rs

@@ -141,6 +141,55 @@ impl JsEmitter {
             }
         }
 
+        // Phase 1b: Emit runtime refinement guards
+        // Collect type aliases from program
+        let mut type_aliases: std::collections::HashMap<String, &TypeExpr> = std::collections::HashMap::new();
+        for decl in &program.declarations {
+            if let Declaration::TypeAlias(alias) = decl {
+                type_aliases.insert(alias.name.clone(), &alias.definition);
+            }
+        }
+
+        let mut guards_emitted = false;
+        for decl in &program.declarations {
+            if let Declaration::Let(let_decl) = decl {
+                // Skip literals — they're statically checked by the type checker
+                if matches!(let_decl.value,
+                    Expr::IntLit(_) | Expr::FloatLit(_) | Expr::StringLit(_) | Expr::BoolLit(_)
+                ) {
+                    continue;
+                }
+
+                if let Some(ref type_ann) = let_decl.type_annotation {
+                    // Resolve type annotation to find refinement predicate
+                    let resolved = match type_ann {
+                        TypeExpr::Named(name) => type_aliases.get(name).copied(),
+                        TypeExpr::Refined { .. } => Some(type_ann),
+                        _ => None,
+                    };
+
+                    if let Some(TypeExpr::Refined { predicate, .. }) = resolved {
+                        if !guards_emitted {
+                            self.emit_line("");
+                            self.emit_line("// ── Refinement Guards ──");
+                            guards_emitted = true;
+                        }
+                        let type_name = match type_ann {
+                            TypeExpr::Named(n) => n.clone(),
+                            _ => "refined type".to_string(),
+                        };
+                        let js_pred = Self::predicate_to_js(predicate, &let_decl.name);
+                        self.emit_line(&format!(
+                            "if (!({js_pred})) throw new Error(\"Refinement violated: `{name}` must satisfy {type_name}\");",
+                            js_pred = js_pred,
+                            name = let_decl.name,
+                            type_name = type_name,
+                        ));
+                    }
+                }
+            }
+        }
+
         self.emit_line("");
 
         // Phase 2a: Component functions
@@ -981,6 +1030,63 @@ impl JsEmitter {
         None
     }
 
+    /// Convert a predicate expression from a `where` clause into a JavaScript boolean expression.
+    /// The `value` identifier is replaced with `signal_name.value` to read the signal's current value.
+    fn predicate_to_js(expr: &Expr, signal_name: &str) -> String {
+        match expr {
+            Expr::Ident(name) if name == "value" => format!("{}.value", signal_name),
+            Expr::Ident(name) => name.clone(),
+            Expr::IntLit(n) => format!("{}", n),
+            Expr::FloatLit(f) => format!("{}", f),
+            Expr::BoolLit(b) => format!("{}", b),
+            Expr::StringLit(s) => {
+                let text: String = s.segments.iter().map(|seg| match seg {
+                    StringSegment::Literal(l) => l.clone(),
+                    _ => String::new(),
+                }).collect();
+                format!("\"{}\"", text)
+            }
+            Expr::BinOp(left, op, right) => {
+                let l = Self::predicate_to_js(left, signal_name);
+                let r = Self::predicate_to_js(right, signal_name);
+                let op_str = match op {
+                    BinOp::Gt => ">",
+                    BinOp::Gte => ">=",
+                    BinOp::Lt => "<",
+                    BinOp::Lte => "<=",
+                    BinOp::Eq => "===",
+                    BinOp::Neq => "!==",
+                    BinOp::And => "&&",
+                    BinOp::Or => "||",
+                    BinOp::Add => "+",
+                    BinOp::Sub => "-",
+                    BinOp::Mul => "*",
+                    BinOp::Div => "/",
+                    BinOp::Mod => "%",
+                };
+                format!("({} {} {})", l, op_str, r)
+            }
+            Expr::UnaryOp(UnaryOp::Not, inner) => {
+                format!("!({})", Self::predicate_to_js(inner, signal_name))
+            }
+            Expr::UnaryOp(UnaryOp::Neg, inner) => {
+                format!("-({})", Self::predicate_to_js(inner, signal_name))
+            }
+            Expr::Call(name, args) => {
+                let js_args: Vec<String> = args.iter()
+                    .map(|a| Self::predicate_to_js(a, signal_name))
+                    .collect();
+                // Map common predicate functions to JS equivalents
+                match name.as_str() {
+                    "len" => format!("{}.length", js_args.first().unwrap_or(&"null".to_string())),
+                    "contains" if js_args.len() == 2 => format!("{}.includes({})", js_args[0], js_args[1]),
+                    _ => format!("{}({})", name, js_args.join(", ")),
+                }
+            }
+            _ => format!("{}.value", signal_name), // fallback
+        }
+    }
+
     fn is_signal_ref(&self, expr: &str) -> bool {
         // Must start with a letter/underscore (not a digit) and contain only ident chars
         !expr.is_empty()

compiler/ds-parser/src/ast.rs

@@ -34,6 +34,8 @@ pub enum Declaration {
     Import(ImportDecl),
     /// `export let count = 0`, `export component Card(...) = ...`
     Export(String, Box<Declaration>),
+    /// `type PositiveInt = Int where value > 0`
+    TypeAlias(TypeAliasDecl),
 }
 
 /// `import { Card, Button } from "./components"`
@@ -48,6 +50,7 @@ pub struct ImportDecl {
 #[derive(Debug, Clone)]
 pub struct LetDecl {
     pub name: String,
+    pub type_annotation: Option<TypeExpr>,
     pub value: Expr,
     pub span: Span,
 }
@@ -159,6 +162,19 @@ pub struct Param {
 pub enum TypeExpr {
     Named(String),
     Generic(String, Vec<TypeExpr>),
+    /// Refinement type: `Int where value > 0`
+    Refined {
+        base: Box<TypeExpr>,
+        predicate: Box<Expr>,
+    },
+}
+
+/// `type PositiveInt = Int where value > 0`
+#[derive(Debug, Clone)]
+pub struct TypeAliasDecl {
+    pub name: String,
+    pub definition: TypeExpr,
+    pub span: Span,
 }
 
 /// Expressions — the core of the language.

compiler/ds-parser/src/lexer.rs

@@ -57,6 +57,8 @@ pub enum TokenKind {
     Every,
     Import,
     Export,
+    Type,
+    Where,
 
     // Operators
     Plus,
@@ -334,6 +336,8 @@ impl Lexer {
             "every" => TokenKind::Every,
             "import" => TokenKind::Import,
             "export" => TokenKind::Export,
+            "type" => TokenKind::Type,
+            "where" => TokenKind::Where,
             _ => TokenKind::Ident(ident.clone()),
         };
 

compiler/ds-parser/src/parser.rs

@@ -101,13 +101,14 @@ impl Parser {
             TokenKind::Every => self.parse_every_decl(),
             TokenKind::Import => self.parse_import_decl(),
             TokenKind::Export => self.parse_export_decl(),
+            TokenKind::Type => self.parse_type_alias_decl(),
             // Expression statement: `log("hello")`, `push(items, x)`
             TokenKind::Ident(_) => {
                 let expr = self.parse_expr()?;
                 Ok(Declaration::ExprStatement(expr))
             }
             _ => Err(self.error(format!(
-                "expected declaration (let, view, effect, on, component, route, constrain, stream, every), got {:?}",
+                "expected declaration (let, view, effect, on, component, route, constrain, stream, every, type), got {:?}",
                 self.peek()
             ))),
         }
@@ -208,16 +209,80 @@ impl Parser {
         let line = self.current_token().line;
         self.advance(); // consume 'let'
         let name = self.expect_ident()?;
+
+        // Optional type annotation: `let name: Type = value`
+        let type_annotation = if self.check(&TokenKind::Colon) {
+            self.advance(); // consume ':'
+            Some(self.parse_type_expr()?)
+        } else {
+            None
+        };
+
         self.expect(&TokenKind::Eq)?;
         let value = self.parse_expr()?;
 
         Ok(Declaration::Let(LetDecl {
             name,
+            type_annotation,
             value,
             span: Span { start: 0, end: 0, line },
         }))
     }
 
+    /// Parse a type alias: `type PositiveInt = Int where value > 0`
+    fn parse_type_alias_decl(&mut self) -> Result<Declaration, ParseError> {
+        let line = self.current_token().line;
+        self.advance(); // consume 'type'
+        let name = self.expect_ident()?;
+        self.expect(&TokenKind::Eq)?;
+        let definition = self.parse_type_expr()?;
+
+        Ok(Declaration::TypeAlias(TypeAliasDecl {
+            name,
+            definition,
+            span: Span { start: 0, end: 0, line },
+        }))
+    }
+
+    /// Parse a type expression: `Int`, `Array<String>`, `Int where value > 0`
+    fn parse_type_expr(&mut self) -> Result<TypeExpr, ParseError> {
+        // Parse base type name
+        let name = match self.peek().clone() {
+            TokenKind::Ident(n) => { self.advance(); n }
+            _ => return Err(self.error(format!("expected type name, got {:?}", self.peek()))),
+        };
+
+        // Optional generic params: `<Type1, Type2>`
+        let base = if self.check(&TokenKind::Lt) {
+            self.advance(); // <
+            let mut params = Vec::new();
+            loop {
+                params.push(self.parse_type_expr()?);
+                if self.check(&TokenKind::Comma) {
+                    self.advance();
+                } else {
+                    break;
+                }
+            }
+            self.expect(&TokenKind::Gt)?;
+            TypeExpr::Generic(name, params)
+        } else {
+            TypeExpr::Named(name)
+        };
+
+        // Optional `where` predicate
+        if self.check(&TokenKind::Where) {
+            self.advance(); // consume 'where'
+            let predicate = self.parse_expr()?;
+            Ok(TypeExpr::Refined {
+                base: Box::new(base),
+                predicate: Box::new(predicate),
+            })
+        } else {
+            Ok(base)
+        }
+    }
+
     fn parse_view_decl(&mut self) -> Result<Declaration, ParseError> {
         let line = self.current_token().line;
         self.advance(); // consume 'view'
@@ -468,23 +533,8 @@ impl Parser {
         Ok(params)
     }
 
-    fn parse_type_expr(&mut self) -> Result<TypeExpr, ParseError> {
-        let name = self.expect_ident()?;
-        if self.check(&TokenKind::Lt) {
-            self.advance();
-            let mut type_args = Vec::new();
-            while !self.check(&TokenKind::Gt) && !self.is_at_end() {
-                type_args.push(self.parse_type_expr()?);
-                if self.check(&TokenKind::Comma) {
-                    self.advance();
-                }
-            }
-            self.expect(&TokenKind::Gt)?;
-            Ok(TypeExpr::Generic(name, type_args))
-        } else {
-            Ok(TypeExpr::Named(name))
-        }
-    }
+
+
 
     // ── Expressions ─────────────────────────────────────
 

compiler/ds-types/src/checker.rs

@@ -8,8 +8,8 @@
 
 use std::collections::HashMap;
 
-use ds_parser::{Program, Declaration, LetDecl, ViewDecl, Expr, BinOp, UnaryOp};
-use crate::types::{Type, TypeVar, EffectType};
+use ds_parser::{Program, Declaration, LetDecl, ViewDecl, Expr, BinOp, UnaryOp, TypeExpr, TypeAliasDecl};
+use crate::types::{Type, TypeVar, EffectType, Predicate, PredicateExpr};
 use crate::errors::{TypeError, TypeErrorKind};
 
 /// The type checker.
@@ -26,6 +26,8 @@ pub struct TypeChecker {
     substitutions: HashMap<TypeVar, Type>,
     /// Currently inside a view block?
     in_view: bool,
+    /// Type alias registry: name → resolved Type.
+    type_aliases: HashMap<String, Type>,
 }
 
 impl TypeChecker {
@@ -37,6 +39,7 @@ impl TypeChecker {
             next_tv: 0,
             substitutions: HashMap::new(),
             in_view: false,
+            type_aliases: HashMap::new(),
         }
     }
 
@@ -54,22 +57,66 @@ impl TypeChecker {
 
     /// Check an entire program.
     pub fn check_program(&mut self, program: &Program) {
+        // Pass 0: register type aliases
+        for decl in &program.declarations {
+            if let Declaration::TypeAlias(alias) = decl {
+                let resolved = self.resolve_type_expr(&alias.definition);
+                self.type_aliases.insert(alias.name.clone(), resolved);
+            }
+        }
+
         // First pass: register all let declarations
         for decl in &program.declarations {
             if let Declaration::Let(let_decl) = decl {
-                let ty = self.infer_expr(&let_decl.value);
+                let inferred_ty = self.infer_expr(&let_decl.value);
+
+                // If there's a type annotation, resolve and check it
+                if let Some(ref type_ann) = let_decl.type_annotation {
+                    let declared_ty = self.resolve_type_expr(type_ann);
+
+                    // Check base type compatibility
+                    let base_declared = match &declared_ty {
+                        Type::Refined { base, .. } => base.as_ref(),
+                        other => other,
+                    };
+
+                    let base_inferred = inferred_ty.unwrap_reactive();
+                    if *base_declared != *base_inferred
+                        && !matches!(base_declared, Type::Var(_))
+                        && !matches!(base_inferred, Type::Var(_))
+                        && *base_declared != Type::Error
+                        && *base_inferred != Type::Error
+                        // Allow Int/Float coercion
+                        && !(matches!(base_declared, Type::Float) && matches!(base_inferred, Type::Int))
+                    {
+                        self.error(TypeErrorKind::Mismatch {
+                            expected: base_declared.clone(),
+                            found: base_inferred.clone(),
+                            context: format!("in declaration of `{}`", let_decl.name),
+                        });
+                    }
+
+                    // Check refinement predicate if present
+                    if let Type::Refined { predicate, .. } = &declared_ty {
+                        self.check_refinement(
+                            predicate,
+                            &let_decl.value,
+                            &let_decl.name,
+                            type_ann,
+                        );
+                    }
+                }
+
                 // Heuristic: if name ends conventionally or is assigned
                 // a literal, mark as Signal<T>; otherwise just let-bound T.
-                // For now: any `let` with a literal is a source signal;
-                // derivations (expressions involving other identifiers) become Derived.
                 let is_source = matches!(
                     let_decl.value,
                     Expr::IntLit(_) | Expr::FloatLit(_) | Expr::StringLit(_) | Expr::BoolLit(_)
                 );
                 let final_ty = if is_source {
-                    Type::Signal(Box::new(ty))
+                    Type::Signal(Box::new(inferred_ty))
                 } else {
-                    Type::Derived(Box::new(ty))
+                    Type::Derived(Box::new(inferred_ty))
                 };
                 self.env.insert(let_decl.name.clone(), final_ty);
             }
@@ -107,6 +154,165 @@ impl TypeChecker {
         }
     }
 
+    /// Resolve a TypeExpr from the AST into a semantic Type.
+    fn resolve_type_expr(&self, type_expr: &TypeExpr) -> Type {
+        match type_expr {
+            TypeExpr::Named(name) => {
+                // Check type aliases first
+                if let Some(resolved) = self.type_aliases.get(name) {
+                    return resolved.clone();
+                }
+                // Built-in type names
+                match name.as_str() {
+                    "Int" => Type::Int,
+                    "Float" => Type::Float,
+                    "String" => Type::String,
+                    "Bool" => Type::Bool,
+                    "View" => Type::View,
+                    _ => Type::Named(name.clone()),
+                }
+            }
+            TypeExpr::Generic(name, params) => {
+                let resolved_params: Vec<Type> = params.iter()
+                    .map(|p| self.resolve_type_expr(p))
+                    .collect();
+                match name.as_str() {
+                    "Signal" if resolved_params.len() == 1 => {
+                        Type::Signal(Box::new(resolved_params.into_iter().next().unwrap()))
+                    }
+                    "Array" if resolved_params.len() == 1 => {
+                        Type::Array(Box::new(resolved_params.into_iter().next().unwrap()))
+                    }
+                    "Stream" if resolved_params.len() == 1 => {
+                        Type::Stream(Box::new(resolved_params.into_iter().next().unwrap()))
+                    }
+                    _ => Type::Named(name.clone()),
+                }
+            }
+            TypeExpr::Refined { base, predicate } => {
+                let base_type = self.resolve_type_expr(base);
+                let pred = Self::ast_to_predicate(predicate);
+                Type::Refined {
+                    base: Box::new(base_type),
+                    predicate: pred,
+                }
+            }
+        }
+    }
+
+    /// Convert an AST expression (from `where` clause) into a semantic Predicate.
+    fn ast_to_predicate(expr: &Expr) -> Predicate {
+        match expr {
+            Expr::BinOp(left, op, right) => {
+                match op {
+                    BinOp::And => {
+                        let l = Self::ast_to_predicate(left);
+                        let r = Self::ast_to_predicate(right);
+                        Predicate::And(Box::new(l), Box::new(r))
+                    }
+                    BinOp::Or => {
+                        let l = Self::ast_to_predicate(left);
+                        let r = Self::ast_to_predicate(right);
+                        Predicate::Or(Box::new(l), Box::new(r))
+                    }
+                    BinOp::Gt => Predicate::Gt(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    BinOp::Gte => Predicate::Gte(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    BinOp::Lt => Predicate::Lt(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    BinOp::Lte => Predicate::Lte(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    BinOp::Eq => Predicate::Eq(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    BinOp::Neq => Predicate::Neq(
+                        Box::new(Self::ast_to_pred_expr(left)),
+                        Box::new(Self::ast_to_pred_expr(right)),
+                    ),
+                    _ => Predicate::Expr(format!("{:?}", expr)),
+                }
+            }
+            Expr::UnaryOp(UnaryOp::Not, inner) => {
+                Predicate::Not(Box::new(Self::ast_to_predicate(inner)))
+            }
+            _ => Predicate::Expr(format!("{:?}", expr)),
+        }
+    }
+
+    /// Convert an AST expression into a predicate sub-expression.
+    fn ast_to_pred_expr(expr: &Expr) -> PredicateExpr {
+        match expr {
+            Expr::Ident(name) if name == "value" => PredicateExpr::Value,
+            Expr::IntLit(n) => PredicateExpr::IntLit(*n),
+            Expr::FloatLit(f) => PredicateExpr::FloatLit(*f),
+            Expr::StringLit(s) => {
+                let text: String = s.segments.iter().map(|seg| match seg {
+                    ds_parser::StringSegment::Literal(l) => l.clone(),
+                    _ => String::new(),
+                }).collect();
+                PredicateExpr::StringLit(text)
+            }
+            Expr::BoolLit(b) => PredicateExpr::BoolLit(*b),
+            Expr::Call(name, args) => {
+                let pred_args: Vec<PredicateExpr> = args.iter()
+                    .map(|a| Self::ast_to_pred_expr(a))
+                    .collect();
+                PredicateExpr::Call(name.clone(), pred_args)
+            }
+            _ => PredicateExpr::Value, // fallback
+        }
+    }
+
+    /// Check a refinement predicate against a value expression.
+    /// For literals: evaluate statically and report error if violated.
+    /// For dynamic expressions: accept (runtime guard will be emitted by codegen).
+    fn check_refinement(
+        &mut self,
+        predicate: &Predicate,
+        value_expr: &Expr,
+        var_name: &str,
+        type_ann: &TypeExpr,
+    ) {
+        // Try to get a static value from the expression
+        let static_val = match value_expr {
+            Expr::IntLit(n) => Some(PredicateExpr::IntLit(*n)),
+            Expr::FloatLit(f) => Some(PredicateExpr::FloatLit(*f)),
+            _ => None,
+        };
+
+        if let Some(val) = static_val {
+            if let Some(result) = predicate.evaluate_static(&val) {
+                if !result {
+                    // Static violation — compile-time error
+                    let type_name = match type_ann {
+                        TypeExpr::Named(n) => n.clone(),
+                        TypeExpr::Refined { base, .. } => match base.as_ref() {
+                            TypeExpr::Named(n) => format!("{} where ...", n),
+                            _ => "<refined>".to_string(),
+                        },
+                        _ => "<unknown>".to_string(),
+                    };
+                    self.error(TypeErrorKind::RefinementViolation {
+                        type_name,
+                        predicate: predicate.display(),
+                        value: format!("{:?}", value_expr),
+                    });
+                }
+            }
+        }
+        // Dynamic values: accepted (codegen emits runtime guard)
+    }
+
     /// Check a view declaration.
     fn check_view(&mut self, view: &ViewDecl) {
         self.in_view = true;
@@ -498,6 +704,7 @@ mod tests {
         let program = make_program(vec![
             Declaration::Let(LetDecl {
                 name: "count".to_string(),
+                type_annotation: None,
                 value: Expr::IntLit(0),
                 span: span(),
             }),
@@ -516,11 +723,13 @@ mod tests {
         let program = make_program(vec![
             Declaration::Let(LetDecl {
                 name: "count".to_string(),
+                type_annotation: None,
                 value: Expr::IntLit(0),
                 span: span(),
             }),
             Declaration::Let(LetDecl {
                 name: "doubled".to_string(),
+                type_annotation: None,
                 value: Expr::BinOp(
                     Box::new(Expr::Ident("count".to_string())),
                     BinOp::Mul,
@@ -567,6 +776,7 @@ mod tests {
         let program = make_program(vec![
             Declaration::Let(LetDecl {
                 name: "name".to_string(),
+                type_annotation: None,
                 value: Expr::StringLit(ds_parser::StringLit {
                     segments: vec![ds_parser::StringSegment::Literal("hello".to_string())],
                 }),
@@ -596,4 +806,115 @@ mod tests {
         let msg = checker.display_errors();
         assert!(msg.contains("VIEW OUTSIDE BLOCK"));
     }
+
+    #[test]
+    fn test_type_annotation_basic() {
+        let mut checker = TypeChecker::new();
+        let program = make_program(vec![
+            Declaration::Let(LetDecl {
+                name: "count".to_string(),
+                type_annotation: Some(ds_parser::TypeExpr::Named("Int".to_string())),
+                value: Expr::IntLit(42),
+                span: span(),
+            }),
+        ]);
+        checker.check_program(&program);
+        assert!(!checker.has_errors(), "Errors: {}", checker.display_errors());
+    }
+
+    #[test]
+    fn test_type_annotation_mismatch() {
+        let mut checker = TypeChecker::new();
+        let program = make_program(vec![
+            Declaration::Let(LetDecl {
+                name: "count".to_string(),
+                type_annotation: Some(ds_parser::TypeExpr::Named("Int".to_string())),
+                value: Expr::StringLit(ds_parser::StringLit {
+                    segments: vec![ds_parser::StringSegment::Literal("oops".to_string())],
+                }),
+                span: span(),
+            }),
+        ]);
+        checker.check_program(&program);
+        assert!(checker.has_errors());
+        let msg = checker.display_errors();
+        assert!(msg.contains("TYPE MISMATCH"));
+    }
+
+    #[test]
+    fn test_refinement_passes_static() {
+        let mut checker = TypeChecker::new();
+        // let count: Int where value > 0 = 5
+        let program = make_program(vec![
+            Declaration::Let(LetDecl {
+                name: "count".to_string(),
+                type_annotation: Some(ds_parser::TypeExpr::Refined {
+                    base: Box::new(ds_parser::TypeExpr::Named("Int".to_string())),
+                    predicate: Box::new(Expr::BinOp(
+                        Box::new(Expr::Ident("value".to_string())),
+                        BinOp::Gt,
+                        Box::new(Expr::IntLit(0)),
+                    )),
+                }),
+                value: Expr::IntLit(5),
+                span: span(),
+            }),
+        ]);
+        checker.check_program(&program);
+        assert!(!checker.has_errors(), "Errors: {}", checker.display_errors());
+    }
+
+    #[test]
+    fn test_refinement_violation_static() {
+        let mut checker = TypeChecker::new();
+        // let count: Int where value > 0 = -1
+        let program = make_program(vec![
+            Declaration::Let(LetDecl {
+                name: "count".to_string(),
+                type_annotation: Some(ds_parser::TypeExpr::Refined {
+                    base: Box::new(ds_parser::TypeExpr::Named("Int".to_string())),
+                    predicate: Box::new(Expr::BinOp(
+                        Box::new(Expr::Ident("value".to_string())),
+                        BinOp::Gt,
+                        Box::new(Expr::IntLit(0)),
+                    )),
+                }),
+                value: Expr::IntLit(-1),
+                span: span(),
+            }),
+        ]);
+        checker.check_program(&program);
+        assert!(checker.has_errors());
+        let msg = checker.display_errors();
+        assert!(msg.contains("REFINEMENT VIOLATED"), "Expected REFINEMENT VIOLATED, got: {}", msg);
+    }
+
+    #[test]
+    fn test_type_alias_with_refinement() {
+        let mut checker = TypeChecker::new();
+        // type PositiveInt = Int where value > 0
+        // let count: PositiveInt = 5
+        let program = make_program(vec![
+            Declaration::TypeAlias(ds_parser::TypeAliasDecl {
+                name: "PositiveInt".to_string(),
+                definition: ds_parser::TypeExpr::Refined {
+                    base: Box::new(ds_parser::TypeExpr::Named("Int".to_string())),
+                    predicate: Box::new(Expr::BinOp(
+                        Box::new(Expr::Ident("value".to_string())),
+                        BinOp::Gt,
+                        Box::new(Expr::IntLit(0)),
+                    )),
+                },
+                span: span(),
+            }),
+            Declaration::Let(LetDecl {
+                name: "count".to_string(),
+                type_annotation: Some(ds_parser::TypeExpr::Named("PositiveInt".to_string())),
+                value: Expr::IntLit(5),
+                span: span(),
+            }),
+        ]);
+        checker.check_program(&program);
+        assert!(!checker.has_errors(), "Errors: {}", checker.display_errors());
+    }
 }

compiler/ds-types/src/errors.rs

@@ -60,6 +60,18 @@ pub enum TypeErrorKind {
         field: String,
         record_type: Type,
     },
+
+    /// A refinement predicate was violated at compile time.
+    RefinementViolation {
+        type_name: String,
+        predicate: String,
+        value: String,
+    },
+
+    /// Circular type alias definition.
+    TypeAliasCycle {
+        name: String,
+    },
 }
 
 impl TypeError {
@@ -144,6 +156,21 @@ impl TypeError {
                     field
                 ))
             }
+            TypeErrorKind::RefinementViolation { type_name, predicate, value } => {
+                ("REFINEMENT VIOLATED".to_string(), format!(
+                    "The value `{}` does not satisfy the refinement type `{}`.\n\n\
+                     The predicate `{}` is not satisfied.\n\n\
+                     Hint: Ensure the value meets the constraint before assignment.",
+                    value, type_name, predicate
+                ))
+            }
+            TypeErrorKind::TypeAliasCycle { name } => {
+                ("TYPE ALIAS CYCLE".to_string(), format!(
+                    "The type alias `{}` refers to itself, creating an infinite loop.\n\n\
+                     Break the cycle by using a concrete base type.",
+                    name
+                ))
+            }
         };
 
         // Format like Elm

compiler/ds-types/src/lib.rs

@@ -4,5 +4,5 @@ pub mod types;
 pub mod errors;
 
 pub use checker::TypeChecker;
-pub use types::{Type, TypeVar, SignalType, EffectType};
+pub use types::{Type, TypeVar, SignalType, EffectType, Predicate, PredicateExpr};
 pub use errors::{TypeError, TypeErrorKind};

compiler/ds-types/src/types.rs

@@ -57,6 +57,16 @@ pub enum Type {
     /// An unresolved type variable (for inference).
     Var(TypeVar),
 
+    /// Refinement type: base type + predicate constraint.
+    /// `Int where value > 0` becomes `Refined { base: Int, predicate: Gt(Value, IntLit(0)) }`
+    Refined {
+        base: Box<Type>,
+        predicate: Predicate,
+    },
+
+    /// Named type alias reference (resolved during checking).
+    Named(String),
+
     /// Error sentinel (for error recovery).
     Error,
 }
@@ -90,6 +100,139 @@ pub enum EffectType {
     Pure,
 }
 
+/// A semantic predicate for refinement types.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Predicate {
+    Gt(Box<PredicateExpr>, Box<PredicateExpr>),
+    Gte(Box<PredicateExpr>, Box<PredicateExpr>),
+    Lt(Box<PredicateExpr>, Box<PredicateExpr>),
+    Lte(Box<PredicateExpr>, Box<PredicateExpr>),
+    Eq(Box<PredicateExpr>, Box<PredicateExpr>),
+    Neq(Box<PredicateExpr>, Box<PredicateExpr>),
+    And(Box<Predicate>, Box<Predicate>),
+    Or(Box<Predicate>, Box<Predicate>),
+    Not(Box<Predicate>),
+    /// A call like `len(value) > 0` or `contains(value, "@")`
+    Call(String, Vec<PredicateExpr>),
+    /// Raw expression that couldn't be further decomposed
+    Expr(String),
+}
+
+/// Predicate sub-expression.
+#[derive(Debug, Clone, PartialEq)]
+pub enum PredicateExpr {
+    /// The refined value itself (the `value` keyword in `where value > 0`)
+    Value,
+    IntLit(i64),
+    FloatLit(f64),
+    StringLit(String),
+    BoolLit(bool),
+    Call(String, Vec<PredicateExpr>),
+}
+
+impl Predicate {
+    /// Pretty-print the predicate for error messages.
+    pub fn display(&self) -> String {
+        match self {
+            Predicate::Gt(l, r) => format!("{} > {}", l.display(), r.display()),
+            Predicate::Gte(l, r) => format!("{} >= {}", l.display(), r.display()),
+            Predicate::Lt(l, r) => format!("{} < {}", l.display(), r.display()),
+            Predicate::Lte(l, r) => format!("{} <= {}", l.display(), r.display()),
+            Predicate::Eq(l, r) => format!("{} == {}", l.display(), r.display()),
+            Predicate::Neq(l, r) => format!("{} != {}", l.display(), r.display()),
+            Predicate::And(l, r) => format!("{} and {}", l.display(), r.display()),
+            Predicate::Or(l, r) => format!("{} or {}", l.display(), r.display()),
+            Predicate::Not(p) => format!("not {}", p.display()),
+            Predicate::Call(name, args) => {
+                let args_str = args.iter().map(|a| a.display()).collect::<Vec<_>>().join(", ");
+                format!("{}({})", name, args_str)
+            }
+            Predicate::Expr(s) => s.clone(),
+        }
+    }
+
+    /// Try to evaluate the predicate statically with a concrete value.
+    /// Returns Some(true/false) if evaluable, None if dynamic.
+    pub fn evaluate_static(&self, value: &PredicateExpr) -> Option<bool> {
+        match self {
+            Predicate::Gt(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv > rv)
+            }
+            Predicate::Gte(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv >= rv)
+            }
+            Predicate::Lt(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv < rv)
+            }
+            Predicate::Lte(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv <= rv)
+            }
+            Predicate::Eq(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv == rv)
+            }
+            Predicate::Neq(l, r) => {
+                let lv = Self::resolve_expr(l, value)?;
+                let rv = Self::resolve_expr(r, value)?;
+                Some(lv != rv)
+            }
+            Predicate::And(l, r) => {
+                let lv = l.evaluate_static(value)?;
+                let rv = r.evaluate_static(value)?;
+                Some(lv && rv)
+            }
+            Predicate::Or(l, r) => {
+                let lv = l.evaluate_static(value)?;
+                let rv = r.evaluate_static(value)?;
+                Some(lv || rv)
+            }
+            Predicate::Not(p) => {
+                let v = p.evaluate_static(value)?;
+                Some(!v)
+            }
+            _ => None, // calls and raw expressions can't be statically evaluated
+        }
+    }
+
+    fn resolve_expr(expr: &PredicateExpr, value: &PredicateExpr) -> Option<f64> {
+        match expr {
+            PredicateExpr::Value => match value {
+                PredicateExpr::IntLit(n) => Some(*n as f64),
+                PredicateExpr::FloatLit(f) => Some(*f),
+                _ => None,
+            },
+            PredicateExpr::IntLit(n) => Some(*n as f64),
+            PredicateExpr::FloatLit(f) => Some(*f),
+            _ => None,
+        }
+    }
+}
+
+impl PredicateExpr {
+    pub fn display(&self) -> String {
+        match self {
+            PredicateExpr::Value => "value".to_string(),
+            PredicateExpr::IntLit(n) => n.to_string(),
+            PredicateExpr::FloatLit(f) => f.to_string(),
+            PredicateExpr::StringLit(s) => format!("\"{}\"", s),
+            PredicateExpr::BoolLit(b) => b.to_string(),
+            PredicateExpr::Call(name, args) => {
+                let args_str = args.iter().map(|a| a.display()).collect::<Vec<_>>().join(", ");
+                format!("{}({})", name, args_str)
+            }
+        }
+    }
+}
+
 impl Type {
     /// Unwrap the inner type of a Signal or Derived.
     pub fn unwrap_reactive(&self) -> &Type {
@@ -147,6 +290,10 @@ impl Type {
             Type::Spring(inner) => format!("Spring<{}>", inner.display()),
             Type::View => "View".to_string(),
             Type::Var(tv) => format!("?{}", tv.0),
+            Type::Refined { base, predicate } => {
+                format!("{} where {}", base.display(), predicate.display())
+            }
+            Type::Named(name) => name.clone(),
             Type::Error => "<error>".to_string(),
         }
     }

examples/refined-types.ds

@@ -0,0 +1,27 @@
+-- DreamStack Dependent Types Example
+-- Demonstrates refinement types, type aliases, and type annotations
+
+-- Type aliases with refinement predicates
+type PositiveInt = Int where value > 0
+type Percentage  = Float where value >= 0.0
+
+-- Basic type annotations
+let count: Int = 0
+let name: String = "hello"
+
+-- Annotated with refinement type alias
+let priority: PositiveInt = 1
+let progress: Percentage = 75.0
+
+-- Derived values don't need annotation (inferred)
+let doubled = count * 2
+let greeting = "Welcome, {name}!"
+
+view main = column [
+  text "Count: {count}"
+  text "Priority: {priority}"
+  text "Progress: {progress}%"
+  text greeting
+  button "+" { click: count += 1 }
+  button "-" { click: count -= 1 }
+]