diff --git a/nix-js/runtime-ts/src/builtins/conversion.ts b/nix-js/runtime-ts/src/builtins/conversion.ts
index 8a33883..b287faf 100644
--- a/nix-js/runtime-ts/src/builtins/conversion.ts
+++ b/nix-js/runtime-ts/src/builtins/conversion.ts
@@ -5,21 +5,21 @@
 import type { NixValue } from "../types";
 
 export const fromJSON = (e: NixValue): never => {
-  throw "Not implemented: fromJSON";
+  throw new Error("Not implemented: fromJSON");
 };
 
 export const fromTOML = (e: NixValue): never => {
-  throw "Not implemented: fromTOML";
+  throw new Error("Not implemented: fromTOML");
 };
 
 export const toJSON = (e: NixValue): never => {
-  throw "Not implemented: toJSON";
+  throw new Error("Not implemented: toJSON");
 };
 
 export const toXML = (e: NixValue): never => {
-  throw "Not implemented: toXML";
+  throw new Error("Not implemented: toXML");
 };
 
 export const toString = (name: NixValue, s: NixValue): never => {
-  throw "Not implemented: toString";
+  throw new Error("Not implemented: toString");
 };
diff --git a/nix-js/runtime-ts/src/builtins/derivation.ts b/nix-js/runtime-ts/src/builtins/derivation.ts
index 34fc186..f00a9be 100644
--- a/nix-js/runtime-ts/src/builtins/derivation.ts
+++ b/nix-js/runtime-ts/src/builtins/derivation.ts
@@ -1,9 +1,9 @@
 import type { NixValue } from "../types";
 
 export const derivation = (args: NixValue) => {
-  throw "Not implemented: derivation";
+  throw new Error("Not implemented: derivation");
 };
 
 export const derivationStrict = (args: NixValue) => {
-  throw "Not implemented: derivationStrict";
+  throw new Error("Not implemented: derivationStrict");
 };
diff --git a/nix-js/runtime-ts/src/builtins/functional.ts b/nix-js/runtime-ts/src/builtins/functional.ts
index aeadcdb..64e7c6e 100644
--- a/nix-js/runtime-ts/src/builtins/functional.ts
+++ b/nix-js/runtime-ts/src/builtins/functional.ts
@@ -2,8 +2,9 @@
  * Functional programming builtin functions
  */
 
-import type { NixValue } from "../types";
+import { CatchableError, type NixValue } from "../types";
 import { force } from "../thunk";
+import { force_string } from "../type-assert";
 
 export const seq =
   (e1: NixValue) =>
@@ -15,15 +16,15 @@ export const seq =
 export const deepSeq =
   (e1: NixValue) =>
   (e2: NixValue): never => {
-    throw "Not implemented: deepSeq";
+    throw new Error("Not implemented: deepSeq");
   };
 
 export const abort = (s: NixValue): never => {
-  throw `evaluation aborted with the following error message: '${force(s)}'`;
+  throw new Error(`evaluation aborted with the following error message: '${force(s)}'`);
 };
 
 export const throwFunc = (s: NixValue): never => {
-  throw force(s);
+  throw new CatchableError(force_string(s));
 };
 
 export const trace = (e1: NixValue, e2: NixValue): NixValue => {
diff --git a/nix-js/runtime-ts/src/builtins/index.ts b/nix-js/runtime-ts/src/builtins/index.ts
index 9ffe6dd..aa4867e 100644
--- a/nix-js/runtime-ts/src/builtins/index.ts
+++ b/nix-js/runtime-ts/src/builtins/index.ts
@@ -249,7 +249,7 @@ export const builtins: any = {
 
   builtins: create_thunk(() => builtins),
   currentSystem: create_thunk(() => {
-    throw "Not implemented: currentSystem";
+    throw new Error("Not implemented: currentSystem");
   }),
   currentTime: create_thunk(() => Date.now()),
 
diff --git a/nix-js/runtime-ts/src/builtins/io.ts b/nix-js/runtime-ts/src/builtins/io.ts
index 983ca4a..a20ebb1 100644
--- a/nix-js/runtime-ts/src/builtins/io.ts
+++ b/nix-js/runtime-ts/src/builtins/io.ts
@@ -25,39 +25,39 @@ export const importFunc = (path: NixValue): NixValue => {
 export const scopedImport =
   (scope: NixValue) =>
   (path: NixValue): never => {
-    throw "Not implemented: scopedImport";
+    throw new Error("Not implemented: scopedImport");
   };
 
 export const storePath = (args: NixValue): never => {
-  throw "Not implemented: storePath";
+  throw new Error("Not implemented: storePath");
 };
 
 export const fetchClosure = (args: NixValue): never => {
-  throw "Not implemented: fetchClosure";
+  throw new Error("Not implemented: fetchClosure");
 };
 
 export const fetchMercurial = (args: NixValue): never => {
-  throw "Not implemented: fetchMercurial";
+  throw new Error("Not implemented: fetchMercurial");
 };
 
 export const fetchGit = (args: NixValue): never => {
-  throw "Not implemented: fetchGit";
+  throw new Error("Not implemented: fetchGit");
 };
 
 export const fetchTarball = (args: NixValue): never => {
-  throw "Not implemented: fetchTarball";
+  throw new Error("Not implemented: fetchTarball");
 };
 
 export const fetchTree = (args: NixValue): never => {
-  throw "Not implemented: fetchTree";
+  throw new Error("Not implemented: fetchTree");
 };
 
 export const fetchurl = (args: NixValue): never => {
-  throw "Not implemented: fetchurl";
+  throw new Error("Not implemented: fetchurl");
 };
 
 export const readDir = (path: NixValue): never => {
-  throw "Not implemented: readDir";
+  throw new Error("Not implemented: readDir");
 };
 
 export const readFile = (path: NixValue): string => {
@@ -66,7 +66,7 @@ export const readFile = (path: NixValue): string => {
 };
 
 export const readFileType = (path: NixValue): never => {
-  throw "Not implemented: readFileType";
+  throw new Error("Not implemented: readFileType");
 };
 
 export const pathExists = (path: NixValue): boolean => {
@@ -75,27 +75,27 @@ export const pathExists = (path: NixValue): boolean => {
 };
 
 export const path = (args: NixValue): never => {
-  throw "Not implemented: path";
+  throw new Error("Not implemented: path");
 };
 
 export const toFile = (name: NixValue, s: NixValue): never => {
-  throw "Not implemented: toFile";
+  throw new Error("Not implemented: toFile");
 };
 
 export const toPath = (name: NixValue, s: NixValue): never => {
-  throw "Not implemented: toPath";
+  throw new Error("Not implemented: toPath");
 };
 
 export const filterSource = (args: NixValue): never => {
-  throw "Not implemented: filterSource";
+  throw new Error("Not implemented: filterSource");
 };
 
 export const findFile =
   (search: NixValue) =>
   (lookup: NixValue): never => {
-    throw "Not implemented: findFile";
+    throw new Error("Not implemented: findFile");
   };
 
 export const getEnv = (s: NixValue): never => {
-  throw "Not implemented: getEnv";
+  throw new Error("Not implemented: getEnv");
 };
diff --git a/nix-js/runtime-ts/src/builtins/misc.ts b/nix-js/runtime-ts/src/builtins/misc.ts
index 18d752d..e2cab1f 100644
--- a/nix-js/runtime-ts/src/builtins/misc.ts
+++ b/nix-js/runtime-ts/src/builtins/misc.ts
@@ -2,141 +2,155 @@
  * Miscellaneous unimplemented builtin functions
  */
 
-import type { NixValue } from "../types";
+import { force } from "../thunk";
+import { CatchableError } from "../types";
+import type { NixBool, NixStrictValue, NixValue } from "../types";
 
 export const addErrorContext =
   (e1: NixValue) =>
   (e2: NixValue): never => {
-    throw "Not implemented: addErrorContext";
+    throw new Error("Not implemented: addErrorContext");
   };
 
 export const appendContext =
   (e1: NixValue) =>
   (e2: NixValue): never => {
-    throw "Not implemented: appendContext";
+    throw new Error("Not implemented: appendContext");
   };
 
 export const getContext = (s: NixValue): never => {
-  throw "Not implemented: getContext";
+  throw new Error("Not implemented: getContext");
 };
 
 export const hasContext = (s: NixValue): never => {
-  throw "Not implemented: hasContext";
+  throw new Error("Not implemented: hasContext");
 };
 
 export const hashFile =
   (type: NixValue) =>
   (p: NixValue): never => {
-    throw "Not implemented: hashFile";
+    throw new Error("Not implemented: hashFile");
   };
 
 export const hashString =
   (type: NixValue) =>
   (p: NixValue): never => {
-    throw "Not implemented: hashString";
+    throw new Error("Not implemented: hashString");
   };
 
 export const convertHash = (args: NixValue): never => {
-  throw "Not implemented: convertHash";
+  throw new Error("Not implemented: convertHash");
 };
 
 export const unsafeDiscardOutputDependency = (s: NixValue): never => {
-  throw "Not implemented: unsafeDiscardOutputDependency";
+  throw new Error("Not implemented: unsafeDiscardOutputDependency");
 };
 
 export const unsafeDiscardStringContext = (s: NixValue): never => {
-  throw "Not implemented: unsafeDiscardStringContext";
+  throw new Error("Not implemented: unsafeDiscardStringContext");
 };
 
 export const unsafeGetAttrPos = (s: NixValue): never => {
-  throw "Not implemented: unsafeGetAttrPos";
+  throw new Error("Not implemented: unsafeGetAttrPos");
 };
 
 export const addDrvOutputDependencies = (s: NixValue): never => {
-  throw "Not implemented: addDrvOutputDependencies";
+  throw new Error("Not implemented: addDrvOutputDependencies");
 };
 
 export const compareVersions =
   (s1: NixValue) =>
   (s2: NixValue): never => {
-    throw "Not implemented: compareVersions";
+    throw new Error("Not implemented: compareVersions");
   };
 
 export const dirOf = (s: NixValue): never => {
-  throw "Not implemented: dirOf";
+  throw new Error("Not implemented: dirOf");
 };
 
 export const flakeRefToString = (attrs: NixValue): never => {
-  throw "Not implemented: flakeRefToString";
+  throw new Error("Not implemented: flakeRefToString");
 };
 
 export const functionArgs = (f: NixValue): never => {
-  throw "Not implemented: functionArgs";
+  throw new Error("Not implemented: functionArgs");
 };
 
 export const genericClosure = (args: NixValue): never => {
-  throw "Not implemented: genericClosure";
+  throw new Error("Not implemented: genericClosure");
 };
 
 export const getFlake = (attrs: NixValue): never => {
-  throw "Not implemented: getFlake";
+  throw new Error("Not implemented: getFlake");
 };
 
 export const match =
   (regex: NixValue) =>
   (str: NixValue): never => {
-    throw "Not implemented: match";
+    throw new Error("Not implemented: match");
   };
 
 export const outputOf =
   (drv: NixValue) =>
   (out: NixValue): never => {
-    throw "Not implemented: outputOf";
+    throw new Error("Not implemented: outputOf");
   };
 
 export const parseDrvName = (s: NixValue): never => {
-  throw "Not implemented: parseDrvName";
+  throw new Error("Not implemented: parseDrvName");
 };
 
 export const parseFlakeName = (s: NixValue): never => {
-  throw "Not implemented: parseFlakeName";
+  throw new Error("Not implemented: parseFlakeName");
 };
 
 export const parseFlakeRef = (s: NixValue): never => {
-  throw "Not implemented: parseFlakeRef";
+  throw new Error("Not implemented: parseFlakeRef");
 };
 
 export const placeholder = (output: NixValue): never => {
-  throw "Not implemented: placeholder";
+  throw new Error("Not implemented: placeholder");
 };
 
 export const replaceStrings =
   (from: NixValue) =>
   (to: NixValue) =>
   (s: NixValue): never => {
-    throw "Not implemented: replaceStrings";
+    throw new Error("Not implemented: replaceStrings");
   };
 
 export const split = (regex: NixValue, str: NixValue): never => {
-  throw "Not implemented: split";
+  throw new Error("Not implemented: split");
 };
 
 export const splitVersion = (s: NixValue): never => {
-  throw "Not implemented: splitVersion";
+  throw new Error("Not implemented: splitVersion");
 };
 
 export const traceVerbose = (e1: NixValue, e2: NixValue): never => {
-  throw "Not implemented: traceVerbose";
+  throw new Error("Not implemented: traceVerbose");
 };
 
-export const tryEval =
-  (e1: NixValue) =>
-  (e2: NixValue): never => {
-    throw "Not implemented: tryEval";
-  };
+export const tryEval = (e: NixValue): { success: NixBool; value: NixStrictValue } => {
+  try {
+    return {
+      success: true,
+      value: force(e),
+    };
+  } catch (err) {
+    if (err instanceof CatchableError) {
+      return {
+        success: false,
+        value: false,
+      };
+    } else {
+      throw err;
+    }
+  }
+};
 
 export const zipAttrsWith =
   (f: NixValue) =>
   (list: NixValue): never => {
-    throw "Not implemented: zipAttrsWith";
+    throw new Error("Not implemented: zipAttrsWith");
   };
diff --git a/nix-js/runtime-ts/src/builtins/type-check.ts b/nix-js/runtime-ts/src/builtins/type-check.ts
index 943e97c..42c6b85 100644
--- a/nix-js/runtime-ts/src/builtins/type-check.ts
+++ b/nix-js/runtime-ts/src/builtins/type-check.ts
@@ -10,6 +10,7 @@ import type {
   NixInt,
   NixList,
   NixNull,
+  NixStrictValue,
   NixString,
   NixValue,
 } from "../types";
@@ -39,7 +40,7 @@ export const isList = (e: NixValue): e is NixList => Array.isArray(force(e));
 export const isNull = (e: NixValue): e is NixNull => force(e) === null;
 
 export const isPath = (e: NixValue): never => {
-  throw "Not implemented: isPath";
+  throw new Error("Not implemented: isPath");
 };
 
 export const isString = (e: NixValue): e is NixString => typeof force(e) === "string";
diff --git a/nix-js/runtime-ts/src/helpers.ts b/nix-js/runtime-ts/src/helpers.ts
index abc42cc..545ddd4 100644
--- a/nix-js/runtime-ts/src/helpers.ts
+++ b/nix-js/runtime-ts/src/helpers.ts
@@ -60,7 +60,7 @@ export const select_with_default = (obj: NixValue, key: NixValue, default_val: N
 
 export const has_attr = (obj: NixValue, attrpath: NixValue[]): NixBool => {
   if (!isAttrs(obj)) {
-    return false
+    return false;
   }
   let attrs = obj;
 
diff --git a/nix-js/runtime-ts/src/thunk.ts b/nix-js/runtime-ts/src/thunk.ts
index 7816b5c..488f904 100644
--- a/nix-js/runtime-ts/src/thunk.ts
+++ b/nix-js/runtime-ts/src/thunk.ts
@@ -3,7 +3,7 @@
  * Implements thunks for lazy evaluation of Nix expressions
  */
 
-import type { NixValue, NixThunkInterface } from "./types";
+import type { NixValue, NixThunkInterface, NixStrictValue } from "./types";
 
 /**
  * Symbol used to mark objects as thunks
@@ -20,12 +20,12 @@ export const IS_THUNK = Symbol("is_thunk");
 export class NixThunk implements NixThunkInterface {
   [key: symbol]: any;
   readonly [IS_THUNK] = true as const;
-  func: (() => NixValue) | null;
-  result: Exclude<NixValue, NixThunkInterface> | null;
+  func: (() => NixValue) | undefined;
+  result: NixStrictValue | undefined;
 
   constructor(func: () => NixValue) {
     this.func = func;
-    this.result = null;
+    this.result = undefined;
   }
 }
 
@@ -46,20 +46,20 @@ export const is_thunk = (value: unknown): value is NixThunkInterface => {
  * @param value - Value to force (may be a thunk)
  * @returns The forced/evaluated value
  */
-export const force = (value: NixValue): Exclude<NixValue, NixThunkInterface> => {
+export const force = (value: NixValue): NixStrictValue => {
   if (!is_thunk(value)) {
     return value;
   }
 
   // Already evaluated - return cached result
-  if (value.func === null) {
+  if (value.func === undefined) {
     return value.result!;
   }
 
   // Evaluate and cache
   const result = force(value.func());
   value.result = result;
-  value.func = null;
+  value.func = undefined;
 
   return result;
 };
diff --git a/nix-js/runtime-ts/src/types.ts b/nix-js/runtime-ts/src/types.ts
index e4644a3..8c1f039 100644
--- a/nix-js/runtime-ts/src/types.ts
+++ b/nix-js/runtime-ts/src/types.ts
@@ -2,6 +2,8 @@
  * Core TypeScript type definitions for nix-js runtime
  */
 
+import { IS_THUNK } from "./thunk";
+
 // Nix primitive types
 export type NixInt = bigint;
 export type NixFloat = number;
@@ -20,9 +22,9 @@ export type NixFunction = (...args: any[]) => any;
  * Thunks delay evaluation until forced
  */
 export interface NixThunkInterface {
-  readonly [key: symbol]: true; // IS_THUNK marker
-  func: (() => NixValue) | null;
-  result: Exclude<NixValue, NixThunkInterface> | null;
+  readonly [IS_THUNK]: true;
+  func: (() => NixValue) | undefined;
+  result: NixStrictValue | undefined;
 }
 
 // Union of all Nix primitive types
@@ -34,6 +36,18 @@ export type NixPrimitive = NixNull | NixBool | NixInt | NixFloat | NixString;
  */
 export type NixValue = NixPrimitive | NixList | NixAttrs | NixFunction | NixThunkInterface;
 
+export type NixStrictValue = Exclude<NixValue, NixThunkInterface>;
+
+/**
+ * CatchableError: Error type thrown by `builtins.throw`
+ * This can be caught by `builtins.tryEval`
+ */
+export class CatchableError extends Error {
+  constructor(msg: string) {
+    super(msg);
+  }
+}
+
 // Operator function signatures
 export type BinaryOp<T = NixValue, U = NixValue, R = NixValue> = (a: T, b: U) => R;
 export type UnaryOp<T = NixValue, R = NixValue> = (a: T) => R;
diff --git a/nix-js/src/runtime.rs b/nix-js/src/runtime.rs
index 83b6a35..7cf1cad 100644
--- a/nix-js/src/runtime.rs
+++ b/nix-js/src/runtime.rs
@@ -3,6 +3,7 @@ use std::pin::Pin;
 use std::sync::Once;
 
 use deno_core::{Extension, ExtensionFileSource, JsRuntime, OpDecl, OpState, RuntimeOptions, v8};
+use deno_error::JsErrorClass;
 
 use crate::codegen::{CodegenContext, Compile};
 use crate::context::{CtxPtr, PathDropGuard};
@@ -44,12 +45,12 @@ mod private {
     pub struct SimpleErrorWrapper(pub(crate) String);
     impl std::fmt::Display for SimpleErrorWrapper {
         fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            std::fmt::Debug::fmt(self, f)
+            std::fmt::Display::fmt(&self.0, f)
         }
     }
     impl std::error::Error for SimpleErrorWrapper {}
 
-    js_error_wrapper!(SimpleErrorWrapper, NixError, "EvalError");
+    js_error_wrapper!(SimpleErrorWrapper, NixError, "Error");
 
     impl From<String> for NixError {
         fn from(value: String) -> Self {
@@ -71,10 +72,13 @@ fn op_import(state: &mut OpState, #[string] path: String) -> std::result::Result
     let ctx = unsafe { ptr.as_mut() };
 
     let current_dir = ctx.get_current_dir();
-    let absolute_path = current_dir
+    let mut absolute_path = current_dir
         .join(&path)
         .canonicalize()
         .map_err(|e| format!("Failed to resolve path {}: {}", path, e))?;
+    if absolute_path.is_dir() {
+        absolute_path.push("default.nix")
+    }
 
     let mut guard = PathDropGuard::new(absolute_path.clone(), ctx);
     let ctx = guard.as_ctx();
@@ -175,7 +179,7 @@ impl Runtime {
         let global_value = self
             .js_runtime
             .execute_script("<eval>", script)
-            .map_err(|e| Error::eval_error(format!("Execution error: {:?}", e)))?;
+            .map_err(|e| Error::eval_error(format!("{}", e.get_message())))?;
 
         // Retrieve scope from JsRuntime
         deno_core::scope!(scope, self.js_runtime);