diff -r dd3050227a84 -r 513b3ef277a3 rust/rhg/src/ui.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/rhg/src/ui.rs	Fri Jun 05 09:01:35 2020 +0200
@@ -0,0 +1,54 @@
+use std::io;
+use std::io::Write;
+
+pub struct Ui {}
+
+/// The kind of user interface error
+pub enum UiError {
+    /// The standard output stream cannot be written to
+    StdoutError(io::Error),
+    /// The standard error stream cannot be written to
+    StderrError(io::Error),
+}
+
+/// The commandline user interface
+impl Ui {
+    pub fn new() -> Self {
+        Ui {}
+    }
+
+    /// Write bytes to stdout
+    pub fn write_stdout(&self, bytes: &[u8]) -> Result<(), UiError> {
+        let mut stdout = io::stdout();
+
+        self.write_stream(&mut stdout, bytes)
+            .or_else(|e| self.into_stdout_error(e))?;
+
+        stdout.flush().or_else(|e| self.into_stdout_error(e))
+    }
+
+    fn into_stdout_error(&self, error: io::Error) -> Result<(), UiError> {
+        self.write_stderr(
+            &[b"abort: ", error.to_string().as_bytes(), b"\n"].concat(),
+        )?;
+        Err(UiError::StdoutError(error))
+    }
+
+    /// Write bytes to stderr
+    pub fn write_stderr(&self, bytes: &[u8]) -> Result<(), UiError> {
+        let mut stderr = io::stderr();
+
+        self.write_stream(&mut stderr, bytes)
+            .or_else(|e| Err(UiError::StderrError(e)))?;
+
+        stderr.flush().or_else(|e| Err(UiError::StderrError(e)))
+    }
+
+    fn write_stream(
+        &self,
+        stream: &mut impl Write,
+        bytes: &[u8],
+    ) -> Result<(), io::Error> {
+        stream.write_all(bytes)
+    }
+}