--- a/src/formats/writelog.rs	Sun Jan 31 14:05:37 2016 +0000
+++ b/src/formats/writelog.rs	Sun Jan 31 14:11:06 2016 +0000
@@ -32,8 +32,8 @@
 /// files are indexed by IDX files describing offset and length of single entries,
 /// which is why we don't need length prefixes here.
 ///
-pub struct WriteLogWriter {
-    dest: Box<Write>,
+pub struct WriteLogWriter<Sink: Write + Sized> {
+    dest: Sink,
 
     current_length: u64,
     records_written: u32,
@@ -59,9 +59,9 @@
     val
 }
 
-impl WriteLogWriter {
+impl<Sink: Write + Sized> WriteLogWriter<Sink> {
     /// Return a new WriteLog that writes to dest
-    pub fn new(dest: Box<Write>) -> WriteLogWriter {
+    pub fn new(dest: Sink) -> WriteLogWriter<Sink> {
         WriteLogWriter {
             dest: dest,
             current_length: 0,
@@ -70,14 +70,14 @@
     }
 
     /// Opens a WriteLog for writing. Truncates a file if append == false.
-    pub fn new_to_file(file: &String, append: bool) -> io::Result<WriteLogWriter> {
+    pub fn new_to_file(file: &String, append: bool) -> io::Result<WriteLogWriter<fs::File>> {
         fs::OpenOptions::new()
             .create(true)
             .write(true)
             .append(append)
             .truncate(!append)
             .open(file)
-            .map(move |f| WriteLogWriter::new(Box::new(f)))
+            .map(move |f| WriteLogWriter::new(f))
     }
 
     /// Return how many (bytes,records) have been written.
@@ -85,7 +85,7 @@
         (self.current_length, self.records_written)
     }
 }
-impl Write for WriteLogWriter {
+impl<Sink: Write + Sized> Write for WriteLogWriter<Sink> {
     fn write(&mut self, buf: &[u8]) -> Result<usize> {
         // BUG: May not account the length in a correct way if the length prefix
         // is written, but not the record.
@@ -107,6 +107,9 @@
     }
 }
 
+/// Like LinesSinkGenerator, opens new WriteLog sinks based on a base path.
+/// The framework-supplied suffices are appended directly to the base path given
+/// to new(). (E.g. base = `/a/b/c`, input = `_shard1` => `/a/b/c_shard1`)
 pub struct WriteLogGenerator {
     base: String,
 }
@@ -118,11 +121,11 @@
 }
 
 impl MRSinkGenerator for WriteLogGenerator {
-    type Sink = WriteLogWriter;
+    type Sink = WriteLogWriter<fs::File>;
     fn new_output(&mut self, suffix: &String) -> Self::Sink {
         let mut path = self.base.clone();
         path.push_str(&suffix[..]);
-        let writer = WriteLogWriter::new_to_file(&path, false);
+        let writer = WriteLogWriter::<fs::File>::new_to_file(&path, false);
         match writer {
             Err(e) => panic!("Could not open {}: {}", path, e),
             Ok(w) => w
@@ -384,7 +387,7 @@
                                     .bytes()
                                     .collect();
 
-        match WriteLogWriter::new_to_file(&String::from("bench_file.wlg"), false) {
+        match WriteLogWriter::<fs::File>::new_to_file(&String::from("bench_file.wlg"), false) {
             Err(e) => panic!("{}", e),
             Ok(ref mut writer) => {
                 let start = PreciseTime::now();