diff --git a/src/langserver/completion.rs b/src/langserver/completion.rs
index 2432dfd8..71da20c1 100644
--- a/src/langserver/completion.rs
+++ b/src/langserver/completion.rs
@@ -8,7 +8,7 @@ use dm::ast::PathOp;
 use dm::annotation::Annotation;
 use dm::objtree::{TypeRef, TypeVar, TypeProc, ProcValue};
 
-use {Engine, Span, io, is_constructor_name, ignore_root};
+use {Engine, Span, is_constructor_name, ignore_root};
 use symbol_search::contains;
 
 pub fn item_var(ty: TypeRef, name: &str, var: &TypeVar) -> CompletionItem {
@@ -126,7 +126,7 @@ pub fn combine_tree_path<'a, I>(iter: &I, mut absolute: bool, mut parts: &'a [St
     prefix_parts.iter().chain(parts).map(|x| &**x)
 }
 
-impl<'a, W: io::ResponseWrite> Engine<'a, W> {
+impl<'a> Engine<'a> {
     pub fn follow_type_path<'b, I>(&'b self, iter: &I, mut parts: &'b [(PathOp, String)]) -> Option<TypePathResult<'b>>
     where
         I: Iterator<Item = (Span, &'a Annotation)> + Clone,
diff --git a/src/langserver/io/mod.rs b/src/langserver/io/mod.rs
index bd4dc590..ad3221e7 100644
--- a/src/langserver/io/mod.rs
+++ b/src/langserver/io/mod.rs
@@ -1,15 +1,59 @@
-//! Pluggable backends for input/output handling.
+//! I/O backend for standard targets.
+//!
+//! JSON-RPC over stdin/stdout with Content-Length headers.
 
-pub trait RequestRead {
-    fn read(&self) -> Option<String>;
+use std::io::{self, Read, Write};
+
+pub fn run_forever<F: FnMut(&str)>(mut f: F) -> ! {
+    loop {
+        let message = read().expect("request bad read");
+        f(&message);
+    }
 }
 
-pub trait ResponseWrite {
-    fn write(&self, output: String);
+fn read() -> Option<String> {
+    macro_rules! check {
+        ($exp:expr) => {
+            match $exp {
+                Ok(x) => x,
+                Err(e) => {
+                    eprintln!("{:?}", e);
+                    return None;
+                }
+            }
+        };
+    }
+
+    // read the content-length
+    let mut buffer = String::new();
+    check!(io::stdin().read_line(&mut buffer));
+    if buffer.is_empty() {
+        return None;
+    }
+    let size = {
+        let parts: Vec<&str> = buffer.split(' ').collect();
+        if parts.len() != 2 {
+            return None;
+        }
+        if !parts[0].eq_ignore_ascii_case("content-length:") {
+            return None;
+        }
+        check!(usize::from_str_radix(parts[1].trim(), 10))
+    };
+
+    // skip blank line
+    buffer.clear();
+    check!(io::stdin().read_line(&mut buffer));
+
+    // read content
+    let mut content = vec![0; size];
+    check!(io::stdin().read_exact(&mut content));
+    Some(check!(String::from_utf8(content)))
 }
 
-#[cfg_attr(target_arch="wasm32", path="wasm.rs")]
-#[cfg_attr(not(target_arch="wasm32"), path="stdio.rs")]
-mod system;
-
-pub use self::system::*;
+pub fn write(output: String) {
+    let stdout = io::stdout();
+    let mut stdout_lock = stdout.lock();
+    write!(stdout_lock, "Content-Length: {}\r\n\r\n{}", output.len(), output).unwrap();
+    stdout_lock.flush().unwrap();
+}
diff --git a/src/langserver/io/stdio.rs b/src/langserver/io/stdio.rs
deleted file mode 100644
index affb5ced..00000000
--- a/src/langserver/io/stdio.rs
+++ /dev/null
@@ -1,70 +0,0 @@
-//! I/O backend for standard targets.
-//!
-//! JSON-RPC over stdin/stdout with Content-Length headers.
-
-use Engine;
-use io::{RequestRead, ResponseWrite};
-use std::io::{self, Read, Write};
-
-pub fn io_main() {
-    let stdio = StdIo;
-    let context = Default::default();
-    let mut engine = Engine::new(&stdio, &context);
-    loop {
-        let message = stdio.read().expect("request bad read");
-        engine.handle_input(&message);
-    }
-}
-
-struct StdIo;
-
-impl RequestRead for StdIo {
-    fn read(&self) -> Option<String> {
-        macro_rules! check {
-            ($exp:expr) => {
-                match $exp {
-                    Ok(x) => x,
-                    Err(e) => {
-                        eprintln!("{:?}", e);
-                        return None;
-                    }
-                }
-            };
-        }
-
-        // read the content-length
-        let mut buffer = String::new();
-        check!(io::stdin().read_line(&mut buffer));
-        if buffer.is_empty() {
-            return None;
-        }
-        let size = {
-            let parts: Vec<&str> = buffer.split(' ').collect();
-            if parts.len() != 2 {
-                return None;
-            }
-            if !parts[0].eq_ignore_ascii_case("content-length:") {
-                return None;
-            }
-            check!(usize::from_str_radix(parts[1].trim(), 10))
-        };
-
-        // skip blank line
-        buffer.clear();
-        check!(io::stdin().read_line(&mut buffer));
-
-        // read content
-        let mut content = vec![0; size];
-        check!(io::stdin().read_exact(&mut content));
-        Some(check!(String::from_utf8(content)))
-    }
-}
-
-impl ResponseWrite for StdIo {
-    fn write(&self, output: String) {
-        let stdout = io::stdout();
-        let mut stdout_lock = stdout.lock();
-        write!(stdout_lock, "Content-Length: {}\r\n\r\n{}", output.len(), output).unwrap();
-        stdout_lock.flush().unwrap();
-    }
-}
diff --git a/src/langserver/io/wasm.rs b/src/langserver/io/wasm.rs
deleted file mode 100644
index b2968135..00000000
--- a/src/langserver/io/wasm.rs
+++ /dev/null
@@ -1,44 +0,0 @@
-//! I/O backend for WebAssembly target.
-//!
-//! `main()` creates an engine that will respond to calls to `handle_input()`,
-//! which will then call `handle_output()` with output messages.
-#![allow(unsafe_code)]
-
-use Engine;
-use super::ResponseWrite;
-
-pub fn io_main() {
-    let wasmio = Box::leak(Box::new(WasmIo));
-    let context = Box::leak(Box::new(Default::default()));
-    let engine = Box::new(Engine::new(wasmio, context));
-    unsafe {
-        ENGINE_PTR = Box::into_raw(engine);
-        return_into_js();
-    }
-}
-
-static mut ENGINE_PTR: *mut Engine<WasmIo> = 0 as *mut Engine<WasmIo>;
-
-#[no_mangle]
-pub unsafe extern fn handle_input(ptr: *const u8, len: usize) {
-    assert!(!ENGINE_PTR.is_null());
-    let engine = &mut *ENGINE_PTR;
-    let slice = std::slice::from_raw_parts(ptr, len);
-    let text = std::str::from_utf8(slice).expect("input is not utf-8");
-    engine.handle_input(text);
-}
-
-struct WasmIo;
-
-impl ResponseWrite for WasmIo {
-    fn write(&self, output: String) {
-        unsafe {
-            handle_output(output.as_ptr(), output.len());
-        }
-    }
-}
-
-extern {
-    fn handle_output(ptr: *const u8, len: usize);
-    fn return_into_js() -> !;
-}
diff --git a/src/langserver/macros.rs b/src/langserver/macros.rs
index 56362ab8..2502e023 100644
--- a/src/langserver/macros.rs
+++ b/src/langserver/macros.rs
@@ -7,7 +7,7 @@ pub mod all_notifications {
 
 macro_rules! handle_method_call {
     ($(on $what:ident(&mut $self:ident, $p:pat) $b:block)*) => {
-        impl<'a, W: io::ResponseWrite> Engine<'a, W> {
+        impl<'a> Engine<'a> {
             fn handle_method_call(&mut self, call: jsonrpc::MethodCall) -> Result<serde_json::Value, jsonrpc::Error> {
                 use langserver::request::*;
 
@@ -57,7 +57,7 @@ macro_rules! handle_method_call {
 
 macro_rules! handle_notification {
     ($(on $what:ident(&mut $self:ident, $p:pat) $b:block)*) => {
-        impl<'a, W: io::ResponseWrite> Engine<'a, W> {
+        impl<'a> Engine<'a> {
             fn handle_notification(&mut self, notification: jsonrpc::Notification) -> Result<(), jsonrpc::Error> {
                 use macros::all_notifications::*;
 
diff --git a/src/langserver/main.rs b/src/langserver/main.rs
index 1ecf0a03..bc8a75ca 100644
--- a/src/langserver/main.rs
+++ b/src/langserver/main.rs
@@ -62,7 +62,9 @@ fn main() {
         Err(e) => eprintln!("dir check failure: {}", e),
     }
 
-    io::io_main();
+    let context = dm::Context::default();
+    let mut engine = Engine::new(&context);
+    io::run_forever(|message| engine.handle_input(message));
 }
 
 const VERSION: Option<jsonrpc::Version> = Some(jsonrpc::Version::V2);
@@ -116,8 +118,7 @@ impl ClientCaps {
     }
 }
 
-struct Engine<'a, W: 'a> {
-    write: &'a W,
+struct Engine<'a> {
     docs: document::DocumentStore,
 
     status: InitStatus,
@@ -135,10 +136,9 @@ struct Engine<'a, W: 'a> {
     client_caps: ClientCaps,
 }
 
-impl<'a, W: io::ResponseWrite> Engine<'a, W> {
-    fn new(write: &'a W, context: &'a dm::Context) -> Self {
+impl<'a> Engine<'a> {
+    fn new(context: &'a dm::Context) -> Self {
         Engine {
-            write,
             docs: Default::default(),
 
             status: InitStatus::Starting,
@@ -165,7 +165,7 @@ impl<'a, W: io::ResponseWrite> Engine<'a, W> {
         T: langserver::notification::Notification,
         T::Params: serde::Serialize,
     {
-        issue_notification::<_, T>(self.write, params)
+        issue_notification::<T>(params)
     }
 
     fn show_message<S>(&mut self, typ: MessageType, message: S) where
@@ -512,8 +512,7 @@ impl<'a, W: io::ResponseWrite> Engine<'a, W> {
                         }
                     }
 
-                    issue_notification::<_, langserver::notification::PublishDiagnostics>(
-                        self.write,
+                    issue_notification::<langserver::notification::PublishDiagnostics>(
                         langserver::PublishDiagnosticsParams {
                             uri: url.to_owned(),
                             diagnostics,
@@ -677,7 +676,7 @@ impl<'a, W: io::ResponseWrite> Engine<'a, W> {
             _ => Response::Batch(outputs),
         };
 
-        self.write.write(serde_json::to_string(&response).expect("response bad to_string"));
+        io::write(serde_json::to_string(&response).expect("response bad to_string"));
     }
 
     fn handle_call(&mut self, call: Call) -> Option<Output> {
@@ -1707,7 +1706,7 @@ fn span_to_range(range: ::std::ops::Range<dm::Location>) -> langserver::Range {
     langserver::Range::new(location_to_position(range.start), location_to_position(range.end))
 }
 
-fn issue_notification<W: io::ResponseWrite, T>(write: &W, params: T::Params)
+fn issue_notification<T>(params: T::Params)
 where
     T: langserver::notification::Notification,
     T::Params: serde::Serialize,
@@ -1718,7 +1717,7 @@ where
         method: T::METHOD.to_owned(),
         params: value_to_params(params),
     }));
-    write.write(serde_json::to_string(&request).expect("notification bad to_string"))
+    io::write(serde_json::to_string(&request).expect("notification bad to_string"))
 }
 
 fn component_to_source(component: dm::Component) -> Option<String> {