Update

src/symbols/dir.rs

@@ -2,7 +2,9 @@ use std::error::Error;
 use std::fmt;
 use std::fs;
 use std::io;
+use std::path::Path;
 
+use resources::Resource;
 use symbols::Symbol;
 
 pub struct Dir<D> where D: AsRef<str> + fmt::Display {
@@ -34,7 +36,19 @@ impl<D> Symbol for Dir<D> where D: AsRef<str> + fmt::Display {
   }
 
   fn execute(&self) -> Result<(), Box<Error>> {
-    fs::create_dir_all(self.path.as_ref()).map_err(|e| Box::new(e) as Box<Error>)
+    fs::create_dir(self.path.as_ref()).map_err(|e| Box::new(e) as Box<Error>)
+  }
+
+  fn get_prerequisites(&self) -> Vec<Resource> {
+    if let Some(parent) = Path::new(self.path.as_ref()).parent() {
+      vec![ Resource::new("dir", parent.to_string_lossy()) ]
+    } else {
+      vec![]
+    }
+  }
+
+  fn provides(&self) -> Option<Vec<Resource>> {
+    Some(vec![ Resource::new("dir", self.path.to_string()) ])
   }
 }
 

src/symbols/git/checkout.rs

@@ -1,8 +1,10 @@
 use std::error::Error;
 use std::fmt;
 use std::io;
+use std::path::Path;
 
 use command_runner::CommandRunner;
+use resources::Resource;
 use symbols::Symbol;
 
 pub struct GitCheckout<'a> {
@@ -67,6 +69,14 @@ impl<'a> Symbol for GitCheckout<'a> {
     try!(self._run_in_target_repo(&["merge", "FETCH_HEAD"]));
     Ok(())
   }
+
+  fn get_prerequisites(&self) -> Vec<Resource> {
+    vec![ Resource::new("dir", Path::new(self.target).parent().unwrap().to_string_lossy()) ]
+  }
+
+  fn provides(&self) -> Option<Vec<Resource>> {
+    Some(vec![ Resource::new("dir", self.target.to_string()) ])
+  }
 }
 
 #[cfg(test)]

src/symbols/hook.rs

@@ -31,6 +31,17 @@ impl<A, B> Symbol for Hook<A, B> where A: Symbol, B: Symbol {
     r.extend(self.b.get_prerequisites().into_iter());
     r
   }
+
+  fn provides(&self) -> Option<Vec<Resource>> {
+    let mut r = vec![];
+    if let Some(provides) = self.a.provides() {
+      r.extend(provides.into_iter());
+    }
+    if let Some(provides) = self.b.provides() {
+      r.extend(provides.into_iter());
+    }
+    if r.len() > 0 { Some(r) } else { None }
+  }
 }
 
 impl<A, B> fmt::Display for Hook<A, B> where A: Symbol, B: Symbol {

src/symbols/list.rs

@@ -32,6 +32,26 @@ impl<'a> Symbol for List<'a> {
     }
     Ok(())
   }
+
+  fn get_prerequisites(&self) -> Vec<Resource> {
+    let mut r = vec![];
+    for symbol in &self.symbols {
+      for req in symbol.get_prerequisites() {
+        if self.provides().map_or(true, |p| !p.contains(&req)) { r.push(req) }
+      }
+    }
+    r
+  }
+
+  fn provides(&self) -> Option<Vec<Resource>> {
+    let mut r = vec![];
+    for symbol in &self.symbols {
+      if let Some(provides) = symbol.provides() {
+        r.extend(provides.into_iter());
+      }
+    }
+    if r.len() > 0 { Some(r) } else { None }
+  }
 }
 
 impl<'a> fmt::Display for List<'a> {

src/symbols/mod.rs

@@ -8,6 +8,9 @@ pub trait Symbol: Display {
   fn get_prerequisites(&self) -> Vec<Resource> {
     vec![]
   }
+  fn provides(&self) -> Option<Vec<Resource>> {
+    None
+  }
 }
 
 pub mod acme;

src/symbols/user.rs

@@ -4,6 +4,7 @@ use std::fmt;
 use std::io::Error as IoError;
 
 use command_runner::CommandRunner;
+use resources::Resource;
 use symbols::Symbol;
 
 #[derive(Debug, PartialEq)]
@@ -108,6 +109,10 @@ impl<'a, E: 'static + Error, A: UserAdder<SubE=E>> Symbol for User<'a, E, A> {
   fn execute(&self) -> Result<(), Box<Error>> {
     self.user_adder.add_user(&*self.user_name).map_err(|e| Box::new(e) as Box<Error>)
   }
+
+  fn provides(&self) -> Option<Vec<Resource>> {
+    Some(vec![Resource::new("user", self.user_name.to_string())])
+  }
 }
 
 pub struct SystemUserAdder<'a> {