diff --git a/src/env.rs b/src/env.rs
index 94dfabf..9f95fa5 100644
--- a/src/env.rs
+++ b/src/env.rs
@@ -13,7 +13,10 @@ pub trait Env {
     fn args (&self) -> Args;
 
     /// Bind a value to a name.
-    fn bind (&mut self, name: &str, value: &str);
+    fn bind <N, V> (&mut self, name: N, value: V)
+    where
+        N: AsRef<str>,
+        V: AsRef<str>;
 
     /// Get a bound value from the env.
     fn get (&self, name: &str) -> Option<String>;
@@ -22,14 +25,21 @@ pub trait Env {
     fn working_dir (&self) -> Result<std::path::PathBuf>;
 
     /// Change the working directory.
-    fn set_working_dir (&mut self, path: &std::path::Path) -> Result<()>;
+    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
+    where
+        P: AsRef<std::path::Path>;
 
     /// Write to stdout.
-    fn stdout (&mut self, data: &str) -> Result<()>;
+    fn stdout <R> (&mut self, data: R) -> Result<()>
+    where
+        R: ToString;
 
     /// Search the `PATH` variable for the `query`. The default implementation
     /// is derived from [`Env::get`].
-    fn search (&self, query: &str) -> Option<exec::Program> {
+    fn search <S> (&self, query: S) -> Option<exec::Program> 
+    where
+        S: AsRef<str>
+    {
         self.get("PATH")?
             .split(':')
             .filter_map(|path| {
@@ -40,7 +50,7 @@ pub trait Env {
             })
             .map(|file| file.path())
             .find_map(|path| {
-                if path.file_name()? == query { 
+                if path.file_name()? == query.as_ref() { 
                     Some (path)
                 } else {
                     None
@@ -64,13 +74,14 @@ pub trait Env {
     /// Create a new environment that inherits this environments'
     /// behavior, except the args of this env are overwritten with
     /// `args`.
-    fn set_args (self, args: Args) -> SetArgs<Self> 
+    fn set_args <A> (self, args: A) -> SetArgs<Self> 
     where
-        Self: Sized
+        Self: Sized,
+        A: IntoIterator<Item = String>,
     {
         SetArgs {
             parent: self, 
-            args,
+            args: Args::from_iter(args),
         }
     }
 
@@ -88,15 +99,24 @@ impl<E: Env> Env for SetArgs<E> {
         self.parent.working_dir()
     }
 
-    fn set_working_dir (&mut self, path: &std::path::Path) -> Result<()> {
+    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
+    where
+        P: AsRef<std::path::Path>
+    {
         self.parent.set_working_dir(path)
     }
 
-    fn search (&self, query: &str) -> Option<exec::Program> {
+    fn search <S> (&self, query: S) -> Option<exec::Program> 
+    where
+        S: AsRef<str>
+    {
         self.parent.search(query)
     }
 
-    fn stdout (&mut self, data: &str) -> Result<()> {
+    fn stdout <R> (&mut self, data: R) -> Result<()>
+    where
+        R: ToString
+    {
         self.parent.stdout(data)
     }
 
@@ -104,7 +124,11 @@ impl<E: Env> Env for SetArgs<E> {
         self.args.clone()
     }
 
-    fn bind (&mut self, name: &str, value: &str) {
+    fn bind <N, V> (&mut self, name: N, value: V)
+    where
+        N: AsRef<str>,
+        V: AsRef<str>,
+    {
         self.parent.bind(name, value);
     }
 
@@ -128,15 +152,24 @@ impl<E: Env> Env for Scope<'_, E> {
         self.parent.working_dir()
     }
 
-    fn set_working_dir (&mut self, path: &std::path::Path) -> Result<()> {
+    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
+    where
+        P: AsRef<std::path::Path>
+    {
         self.parent.set_working_dir(path)
     }
 
-    fn search (&self, query: &str) -> Option<exec::Program> {
+    fn search <S> (&self, query: S) -> Option<exec::Program> 
+    where
+        S: AsRef<str>
+    {
         self.parent.search(query)
     }
 
-    fn stdout (&mut self, data: &str) -> Result<()> {
+    fn stdout <R> (&mut self, data: R) -> Result<()>
+    where
+        R: ToString
+    {
         self.parent.stdout(data)
     }
 
@@ -144,8 +177,15 @@ impl<E: Env> Env for Scope<'_, E> {
         self.parent.args()
     }
 
-    fn bind (&mut self, name: &str, value: &str) {
-        self.bindings.insert(name.to_string(), value.to_string());
+    fn bind <N, V> (&mut self, name: N, value: V)
+    where
+        N: AsRef<str>,
+        V: AsRef<str>,
+    {
+        self.bindings.insert(
+            name.as_ref().to_string(),
+            value.as_ref().to_string()
+        );
     }
 
     fn get (&self, name: &str) -> Option<String> {
@@ -206,16 +246,26 @@ impl Env for Pure {
         std::iter::empty::<String>().collect()
     }
 
-    fn bind (&mut self, name: &str, value: &str) {
-        self.bindings.insert(name.to_string(), value.to_string());
+    fn bind <N, V> (&mut self, name: N, value: V)
+    where
+        N: AsRef<str>,
+        V: AsRef<str>,
+    {
+        self.bindings.insert(
+            name.as_ref().to_string(),
+            value.as_ref().to_string()
+        );
     }
 
     fn get (&self, name: &str) -> Option<String> {
         self.bindings.get(name).cloned()
     }
 
-    fn stdout (&mut self, data: &str) -> Result<()> {
-        self.buf += data;
+    fn stdout <R> (&mut self, data: R) -> Result<()>
+    where
+        R: ToString
+    {
+        self.buf += &data.to_string();
         Ok (())
     }
 
@@ -228,7 +278,12 @@ impl Env for Pure {
         }
     }
 
-    fn set_working_dir (&mut self, path: &std::path::Path) -> Result<()> {
+    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
+    where
+        P: AsRef<std::path::Path>
+    {
+        let path = path.as_ref();
+
         if path.is_dir() && path.exists() {
             Ok (self.cwd = path.to_owned())
         } else {
@@ -247,8 +302,12 @@ impl Env for Inherit {
         std::env::args().skip(1).collect()
     }
 
-    fn bind (&mut self, name: &str, value: &str) {
-        std::env::set_var(name, value);
+    fn bind <N, V> (&mut self, name: N, value: V)
+    where
+        N: AsRef<str>,
+        V: AsRef<str>,
+    {
+        std::env::set_var(name.as_ref(), value.as_ref());
     }
 
     fn get (&self, name: &str) -> Option<String> {
@@ -260,13 +319,24 @@ impl Env for Inherit {
             .map_err(Error::Io)
     }
 
-    fn set_working_dir (&mut self, path: &std::path::Path) -> Result<()> {
+    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
+    where
+        P: AsRef<std::path::Path>
+    {
+        let path = path.as_ref();
+
         std::env::set_current_dir(path)
             .map_err(Error::Io)
     }
 
-    fn stdout (&mut self, data: &str) -> Result<()> {
-        std::io::stdout().write(data.as_bytes())?;
+    fn stdout <R> (&mut self, data: R) -> Result<()>
+    where
+        R: ToString
+    {
+        std::io::stdout().write(
+            data.to_string().as_bytes()
+        )?;
+        
         Ok (())
     }
 }
diff --git a/src/exec.rs b/src/exec.rs
index 9c2a21d..d3a7732 100644
--- a/src/exec.rs
+++ b/src/exec.rs
@@ -15,18 +15,6 @@ pub trait Exec {
 
 }
 
-impl<F> Exec for F 
-where
-    F: Fn (&mut dyn Env) -> Result<()>
-{
-    fn exec <E> (&self, env: &mut E) -> Result<()>
-    where
-        E: Env
-    {
-        self (env)
-    }
-}
-
 /// A shell command.
 pub enum Command {
     Program (Program),
@@ -130,20 +118,30 @@ pub struct NotFound;
 /// ```
 ///
 /// [`FromStr`]: std::str::FromStr
-pub struct Builtin (fn (&mut dyn Env) -> Result<()>);
+pub struct Builtin (BuiltinName);
+
+enum BuiltinName {
+    Disown,
+    Type,
+    Exit,
+    Pwd,
+    Fg,
+    Bg,
+    Cd,
+}
 
 impl std::str::FromStr for Builtin {
     type Err = NotFound;
 
     fn from_str (s: &str) -> Result<Self, Self::Err> {
         let ret = Builtin (match s {
-            "disown" => builtin::disown,
-            "type"   => builtin::r#type,
-            "exit"   => builtin::exit,
-            "pwd"    => builtin::pwd,
-            "fg"     => builtin::fg,
-            "bg"     => builtin::bg,
-            "cd"     => builtin::cd,
+            "disown" => BuiltinName::Disown,
+            "type"   => BuiltinName::Type,
+            "exit"   => BuiltinName::Exit,
+            "pwd"    => BuiltinName::Pwd,
+            "fg"     => BuiltinName::Fg,
+            "bg"     => BuiltinName::Bg,
+            "cd"     => BuiltinName::Cd,
             _ => return Err (NotFound)
         });
 
@@ -156,7 +154,19 @@ impl Exec for Builtin {
     where
         E: Env
     {
-        self.0.exec(env)
+        use BuiltinName::*;
+
+        let f = match self.0 {
+            Disown => builtin::disown,
+            Type   => builtin::r#type,
+            Exit   => builtin::exit,
+            Pwd    => builtin::pwd,
+            Fg     => builtin::fg,
+            Bg     => builtin::bg,
+            Cd     => builtin::cd,
+        };
+
+        f (env)
     }
 }
 
@@ -167,20 +177,20 @@ pub mod builtin {
     use super::{ Env, Result, };
 
     /// Change the current working directory.
-    pub fn cd (env: &mut dyn Env) -> Result<()> {
+    pub fn cd (env: &mut impl Env) -> Result<()> {
         let arg = env.args().next().unwrap();
-        env.set_working_dir(arg.as_ref())
+        env.set_working_dir(arg)
     }
 
     /// Print the current working directory.
-    pub fn pwd (env: &mut dyn Env) -> Result<()> {
+    pub fn pwd (env: &mut impl Env) -> Result<()> {
         let cwd = env.working_dir()?;
         env.stdout(format!("{}", cwd.display()).as_str());
         Ok (())
     }
 
     /// Stop the program. Optionally takes an exit code.
-    pub fn exit (env: &mut dyn Env) -> Result<()> {
+    pub fn exit (env: &mut impl Env) -> Result<()> {
         if let Some (code) = env.args().next().and_then(|s| s.parse::<i32>().ok()) {
             std::process::exit(code)
         } else {
@@ -189,7 +199,7 @@ pub mod builtin {
     }
 
     /// Display information about the type of a command.
-    pub fn r#type (env: &mut dyn Env) -> Result<()> {
+    pub fn r#type (env: &mut impl Env) -> Result<()> {
         use crate::exec::Builtin;
 
         for arg in env.args() {
@@ -210,19 +220,19 @@ pub mod builtin {
     /// Send a job to the background. See also [`Job::bg`].
     ///
     /// [`Job::bg`]: crate::job::Job::bg
-    pub fn bg (_: &mut dyn Env) -> Result<()> {
+    pub fn bg (_: &mut impl Env) -> Result<()> {
         todo!()
     }
 
     /// Bring a job to the foreground. See also [`Job::fg`].
     ///
     /// [`Job::fg`]: crate::job::Job::fg
-    pub fn fg (_: &mut dyn Env) -> Result<()> {
+    pub fn fg (_: &mut impl Env) -> Result<()> {
         todo!()
     }
 
     /// Disown all background jobs.
-    pub fn disown (_: &mut dyn Env) -> Result<()> {
+    pub fn disown (_: &mut impl Env) -> Result<()> {
         todo!()
     }