diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ea8c4bf
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/target
diff --git a/COPYING b/COPYING
new file mode 100644
index 0000000..451bb46
--- /dev/null
+++ b/COPYING
@@ -0,0 +1,27 @@
+Copyright 2019 Chloé Kekoa
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice,
+   this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. Neither the name of the copyright holder nor the names of its contributors
+   may be used to endorse or promote products derived from this software
+   without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..be647b0
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,6 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "contralog"
+version = "0.0.1"
+
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..2d04ada
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "contralog"
+version = "0.0.1"
+license-file = "COPYING"
+
+authors = ["Chloé Kekoa"]
+description = "Composable logging with monoids and contravariant functors."
+homepage = "https://github.com/chloekek/contralog.git"
+documentation = "https://docs.rs/contralog"
+repository = "https://github.com/chloekek/contralog.git"
diff --git a/README b/README
new file mode 100644
index 0000000..61ae1e2
--- /dev/null
+++ b/README
@@ -0,0 +1,11 @@
+Composable logging with monoids and contravariant functors.
+
+If you want to use this library there are at least two ways to obtain it: you
+could depend on it with Cargo by adding contralog as a dependency in
+Cargo.toml, or you could copy the src/lib.rs file into your own project under
+a different name.
+
+When developing this library please use Nix to ensure the build is
+reproducible:
+
+    nix run -ic cargo test
diff --git a/default.nix b/default.nix
new file mode 100644
index 0000000..8ec9a47
--- /dev/null
+++ b/default.nix
@@ -0,0 +1,7 @@
+let
+    pkgs = import ./nix/pkgs.nix {};
+in
+    [
+        pkgs.cargo
+        pkgs.gcc
+    ]
diff --git a/nix/pkgs.nix b/nix/pkgs.nix
new file mode 100644
index 0000000..e5b2a73
--- /dev/null
+++ b/nix/pkgs.nix
@@ -0,0 +1,8 @@
+let
+    tarball = fetchTarball {
+        url = "https://github.com/NixOS/nixpkgs/archive/f5cc5ce8d60fe69c968582434fbfbf8f350555cb.tar.gz";
+        sha256 = "025773zp9hvizwf4frimm7mnr6cydmckw7kayqmik6scisq0mfk5";
+    };
+    config = {};
+in
+    {}: import tarball {inherit config;}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..4dca380
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,247 @@
+//! Composable logging with monoids and contravariant functors.
+//!
+//! A logger is a routine that takes input and has side-effects.
+//! Any routine that has the appropriate type will do.
+//! A logger can be seen as the opposite or dual of an infinite iterator.
+//!
+//! The core trait of this crate is [Logger].
+//! It has only a single method that must be implemented: log.
+//! To log something, pass it to this method.
+//! It is up to the logger to decide what to do with the value.
+//!
+//! Loggers are composable:
+//! given two loggers with compatible types,
+//! a new logger can be created that forwards
+//! its input to both loggers.
+//!
+//! Loggers can also be transformed using
+//! methods such as [map] and [filter].
+//!
+//! [Logger]: trait.Logger.html
+//! [map]: trait.Logger.html#method.map
+//! [filter]: trait.Logger.html#method.filter
+
+use std::convert::Infallible;
+use std::iter;
+use std::marker::PhantomData;
+
+/// A logger is a routine that takes input and has side-effects.
+///
+/// See the documentation for the [contralog](index.html) crate
+/// for a general overview of loggers.
+pub trait Logger<I>
+{
+    type Error;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>;
+
+    /// Create a “by reference” adaptor for this logger.
+    fn by_ref(&mut self) -> &mut Self
+    {
+        self
+    }
+
+    /// Combine two loggers, creating a new logger
+    /// that logs each input to both loggers.
+    fn chain<L>(self, other: L) -> Chain<Self, L>
+        where Self: Sized
+    {
+        Chain{fst: self, snd: other}
+    }
+
+    /// Apply a function to each input and
+    /// pass it to the logger
+    /// only if the function returns true for it.
+    fn filter<F>(self, f: F) -> Filter<Self, F>
+        where Self: Sized, F: FnMut(&I) -> bool
+    {
+        Filter{inner: self, f}
+    }
+
+    /// Apply a function to each input
+    /// before passing it to the logger.
+    fn map<F, B>(self, f: F) -> Map<Self, F, B>
+        where Self: Sized
+    {
+        Map{inner: self, f, _phantom: PhantomData}
+    }
+
+    /// Return a logger that
+    /// silently drops errors reported by this logger.
+    fn safe<E>(self) -> Safe<Self, E>
+        where Self: Sized
+    {
+        Safe{inner: self, _phantom: PhantomData}
+    }
+}
+
+impl<'a, I, L> Logger<I> for &'a mut L
+    where L: Logger<I>
+{
+    type Error = L::Error;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>
+    {
+        (**self).log(item)
+    }
+}
+
+/// Returned from the [Logger::chain](trait.Logger.html#method.chain) method.
+pub struct Chain<L, M>
+{
+    fst: L,
+    snd: M,
+}
+
+impl<I, L, M> Logger<I> for Chain<L, M>
+    where L: Logger<I>, M: Logger<I, Error=L::Error>, I: Clone
+{
+    type Error = L::Error;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>
+    {
+        self.fst.log(item.clone())?;
+        self.snd.log(item)
+    }
+}
+
+/// A logger that ignores all input.
+pub fn empty<I, E>() -> Empty<I, E>
+{
+    Empty{_phantom: PhantomData}
+}
+
+/// Returned from the [empty](fn.empty.html) function.
+pub struct Empty<I, E>
+{
+    _phantom: PhantomData<fn() -> (I, E)>,
+}
+
+impl<I, E> Logger<I> for Empty<I, E>
+{
+    type Error = E;
+    fn log(&mut self, _item: I) -> Result<(), E>
+    {
+        Ok(())
+    }
+}
+
+/// A logger that collects values into a container.
+pub fn extender<C, I>(container: C) -> Extender<C, I>
+{
+    Extender{container, _phantom: PhantomData}
+}
+
+/// Returned from the [extender](fn.extender.html) function.
+pub struct Extender<C, I>
+{
+    pub container: C,
+    _phantom: PhantomData<fn() -> I>,
+}
+
+impl<C, I> Logger<I> for Extender<C, I>
+    where C: Extend<I>
+{
+    type Error = Infallible;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>
+    {
+        let from = iter::once(item);
+        self.container.extend(from);
+        Ok(())
+    }
+}
+
+/// Returned from the [Logger::filter](trait.Logger.html#method.filter) method.
+pub struct Filter<L, F>
+{
+    inner: L,
+    f: F,
+}
+
+impl<I, L, F> Logger<I> for Filter<L, F>
+    where L: Logger<I>, F: FnMut(&I) -> bool
+{
+    type Error = L::Error;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>
+    {
+        if (self.f)(&item) {
+            self.inner.log(item)
+        } else {
+            Ok(())
+        }
+    }
+}
+
+/// Returned from the [Logger::map](trait.Logger.html#method.map) method.
+pub struct Map<L, F, B>
+{
+    inner: L,
+    f: F,
+    _phantom: PhantomData<fn() -> B>,
+}
+
+impl<I, L, F, B> Logger<B> for Map<L, F, B>
+    where L: Logger<I>, F: FnMut(B) -> I
+{
+    type Error = L::Error;
+    fn log(&mut self, item: B) -> Result<(), Self::Error>
+    {
+        let new_item = (self.f)(item);
+        self.inner.log(new_item)
+    }
+}
+
+/// Returned from the [Logger::safe](trait.Logger.html#method.safe) method.
+pub struct Safe<L, E>
+{
+    inner: L,
+    _phantom: PhantomData<fn() -> E>,
+}
+
+impl<I, L, E> Logger<I> for Safe<L, E>
+    where L: Logger<I>
+{
+    type Error = E;
+    fn log(&mut self, item: I) -> Result<(), Self::Error>
+    {
+        let result = self.inner.log(item);
+        drop(result);
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests
+{
+    use super::*;
+
+    #[test]
+    fn test_chain()
+    {
+        let mut fst = extender(Vec::new());
+        let mut snd = extender(Vec::new());
+        let mut thd = fst.by_ref().chain(&mut snd);
+        thd.log(0).unwrap();
+        assert_eq!(&fst.container, &[0]);
+        assert_eq!(&snd.container, &[0]);
+    }
+
+    #[test]
+    fn test_filter()
+    {
+        let mut fst = extender(Vec::new());
+        let mut snd = fst.by_ref().filter(|&i| i >= 0);
+        snd.log(-1).unwrap();
+        snd.log(0).unwrap();
+        snd.log(1).unwrap();
+        assert_eq!(&fst.container, &[0, 1]);
+    }
+
+    #[test]
+    fn test_map()
+    {
+        let mut fst = extender(Vec::new());
+        let mut snd = fst.by_ref().map(|i: i32| i.abs());
+        snd.log(-1).unwrap();
+        snd.log(0).unwrap();
+        snd.log(1).unwrap();
+        assert_eq!(&fst.container, &[1, 0, 1]);
+    }
+}