diff --git a/README.md b/README.md
index fafb6a7..3caba63 100644
--- a/README.md
+++ b/README.md
@@ -3,20 +3,32 @@
 [![CI status](https://github.com/oxalica/rust-overlay/workflows/CI/badge.svg)](https://github.com/oxalica/rust-overlay/actions/workflows/ci.yaml)
 [![sync-channels status](https://github.com/oxalica/rust-overlay/workflows/sync-channels/badge.svg)](https://github.com/oxalica/rust-overlay/actions/workflows/sync-channels.yaml)
 
-*Pure and reproducible* overlay for binary distributed rust toolchains.
-A compatible but better replacement for rust overlay of [nixpkgs-mozilla].
+*Pure and reproducible* packaging of binary distributed rust toolchains.
+A compatible but better replacement for rust overlay of [nixpkgs-mozilla],
+with also non-overlay and [Nix Flake][flake] interfaces (despite the project name).
 
-Hashes of toolchain components are pre-fetched in tree, so the evaluation is *pure* and
-no need to have network access. It also works well with [Nix Flakes](https://nixos.wiki/wiki/Flakes).
+[flake]: https://wiki.nixos.org/wiki/Flakes
+
+For migration from [nixpkgs-mozilla], see [this section](#migration-from-nixpkgs-mozilla).
+
+Features:
+
+- Hashes of toolchain components are pre-fetched in tree, so the evaluation is
+  *pure* and no need to have network access.
+
+- These hashes are auto-updated daily using GitHub Actions.
 
-- The toolchain hashes are auto-updated daily using GitHub Actions.
 - Current oldest supported version is stable 1.29.0 and beta/nightly 2018-09-13
   (which are randomly picked and may change over time).
 
-rust-overlay targets nixos-unstable and supported releases of NixOS, on x86_64-linux.
-They are tested on CI. It may also work on other channels but is not guarenteed.
+- We targets nixos-unstable and supported releases of NixOS, on x86\_64-linux.
+  They are tested on CI. Other platforms and nixpkgs channels may also work but
+  is not guaranteed.
 
-For migration from [nixpkgs-mozilla], see [this section](#migration-from-nixpkgs-mozilla).
+Documentations:
+
+- [API References](docs/reference.md)
+- [Cross compilation](docs/cross_compilation.md)
 
 ## Installation
 
@@ -41,8 +53,6 @@ And then feel free to use it anywhere like
 
 ### Nix Flakes
 
-This repository already has flake support.
-
 **Warning: Only the output `overlay`/`overlays` are currently stable. Use other outputs at your own risk!**
 
 For a quick play, just use `nix shell` to bring the latest stable rust toolchain into scope.
@@ -218,11 +228,6 @@ Running `nix develop` will create a shell with the default beta Rust toolchain i
 
 - There also an cross-compilation example in [`examples/cross-aarch64`].
 
-## More documentations
-
-- [Reference of all public attributes](docs/reference.md)
-- [Cross compilation](docs/cross_compilation.md)
-
 ## License
 
 MIT licensed.
diff --git a/docs/reference.md b/docs/reference.md
index 652bfc0..9d4e907 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -1,85 +1,154 @@
-# `rust-overlay` reference
+# Reference
 
-All public attributes provided by the overlay are below. Fields not defined here are for internal usage.
+We provides two entry interfaces:
+
+1.  Overlay interface, via `default.nix` (non-flake) or
+    `rust-overlay.overlays.default` (flake).
+
+    This provides a [nixpkgs overlay][nixpkgs-overlay] to be used when
+    importing nixpkgs. The overlay will (try its best) not modify any existing
+    attributes of nixpkgs, but adds these new attributes:
+
+    - `rust-bin`,
+      where all main functionalities resides.
+      Its structure is documented in the next section.
+
+    - `latest`, `rustChannelOf`, `rustChannelOfTargets`, `rustChannels`,
+      for compatibility with [nixpkgs-mozilla].
+
+      They are built on top of `rust-bin` with only structure/argument
+      differences, and will provide the same corresponding derivations as
+      `rust-bin`.
+
+[nixpkgs-overlay]: https://wiki.nixos.org/wiki/Overlays
+[nixpkgs-mozilla]: https://github.com/mozilla/nixpkgs-mozilla
+
+2.  Builder interface, via `rust-overlay.lib.mkRustBin` (flake only).
+
+    It is a function with type
+    `{ distRoot ? ... } -> (pkgs: attrset) -> attrset`.
+
+    It returns an attrset with the same structure as `rust-bin` attribute
+    mentioned above, with the argument of an already imported (instantiated)
+    nixpkgs.
+
+    Notably, only the attrset `rust-bin` is returned, so this builder will not
+    give you any nixpkgs-mozilla compatible attributes.
+
+## `rust-bin` structure
 
 ```nix
 {
-  rust-bin = {
-    # The default dist url for fetching.
-    # Override it if you want to use a mirror server.
-    distRoot = "https://static.rust-lang.org/dist";
+  # The default dist url for fetching.
+  #
+  # For the overlay interface, this attribute is overridable. Changing it will
+  # also change all dependent URLs for source derivations (FOD). Useful for
+  # setting mirrors.
+  distRoot = "https://static.rust-lang.org/dist";
 
-    # Select a toolchain and aggregate components by rustup's `rust-toolchain` file format.
-    # See: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
-    fromRustupToolchain = { channel, components ? [], targets ? [] }: «derivation»;
-    # Same as `fromRustupToolchain` but read from a `rust-toolchain` file (legacy one-line string or in TOML).
-    fromRustupToolchainFile = rust-toolchain-file-path: «derivation»;
+  # Select a toolchain and aggregate components by rustup's `rust-toolchain` file format.
+  # See: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
+  fromRustupToolchain = { channel, components ? [], targets ? [] }: «derivation»;
+  # Same as `fromRustupToolchain` but read from a `rust-toolchain` file (legacy one-line string or in TOML).
+  fromRustupToolchainFile = rust-toolchain-file-path: «derivation»;
 
-    # Select the latest nightly toolchain which have specific components or profile available.
-    # This helps nightly users in case of latest nightly may not contains all components they want.
-    #
-    # `selectLatestNightlyWith (toolchain: toolchain.default)` selects the latest nightly toolchain
-    # with all `default` components (rustc, cargo, rustfmt, ...) available.
-    selectLatestNightlyWith = selector: «derivation»;
+  # Select the latest nightly toolchain which have specific components or profile available.
+  # This helps nightly users in case of latest nightly may not contains all components they want.
+  #
+  # `selectLatestNightlyWith (toolchain: toolchain.default)` selects the latest nightly toolchain
+  # with all `default` components (rustc, cargo, rustfmt, ...) available.
+  selectLatestNightlyWith = selector: «derivation»;
 
-    # Custom toolchain from a specific rustc git revision.
-    # This does almost the same thing as `rustup-toolchain-install-master`. (https://crates.io/crates/rustup-toolchain-install-master)
-    # Parameter `components` should be an attrset with component name as key and its SRI hash as value.
-    fromRustcRev = { pname ? …, rev, components, target ? … }: «derivation»;
+  # Custom toolchain from a specific rustc git revision.
+  # This does almost the same thing as `rustup-toolchain-install-master`. (https://crates.io/crates/rustup-toolchain-install-master)
+  # Parameter `components` should be an attrset with component name as key and its SRI hash as value.
+  fromRustcRev = { pname ? …, rev, components, target ? … }: «derivation»;
 
-    stable = {
-      # The latest stable toolchain.
-      latest = {
-        # Profiles, predefined component sets.
-        # See: https://rust-lang.github.io/rustup/concepts/profiles.html
-        minimal = «derivation»;  # Only `cargo`, `rustc` and `rust-std`.
-        default = «derivation»;  # The default profile of `rustup`. Good for general use.
-        complete = «derivation»; # Do not use it. It almost always fails.
+  stable = {
+    # The latest stable toolchain.
+    latest = {
+      # Profiles, predefined component sets.
+      # See: https://rust-lang.github.io/rustup/concepts/profiles.html
+      minimal = «derivation»;  # Only `cargo`, `rustc` and `rust-std`.
+      default = «derivation»;  # The default profile of `rustup`. Good for general use.
+      complete = «derivation»; # Do not use it. It almost always fails.
 
-        # Pre-aggregated package provided by upstream, the most commonly used package in `mozilla-overlay`.
-        # It consists of an uncertain number of components, usually more than the `default` profile of `rustup`
-        # but less than `complete` profile.
-        rust = «derivation»;
+      # Pre-aggregated package provided by upstream, the most commonly used package in `mozilla-overlay`.
+      # It consists of an uncertain number of components, usually more than the `default` profile of `rustup`
+      # but less than `complete` profile.
+      rust = «derivation»;
 
-        # Individial components.
-        rustc = «derivation»;
-        cargo = «derivation»;
-        rust-std = «derivation»;
-        # ... other components
-      };
-      "1.49.0" = { /* toolchain */ };
-      "1.48.0" = { /* toolchain */ };
-      # ... other versions.
+      # Individial components.
+      rustc = «derivation»;
+      cargo = «derivation»;
+      rust-std = «derivation»;
+      # ... other components
     };
-
-    beta = {
-      # The latest beta toolchain.
-      latest = { /* toolchain */ };
-      "2021-01-01" = { /* toolchain */ };
-      "2020-12-30" = { /* toolchain */ };
-      # ... other versions.
-    };
-
-    nightly = {
-      # The latest nightly toolchain.
-      # It is preferred to use `selectLatestNightlyWith` instead of this since
-      # nightly toolchain may have components (like `rustfmt` or `rls`) missing,
-      # making `default` profile unusable.
-      latest = { /* toolchain */ };
-      "2020-12-31" = { /* toolchain */ };
-      "2020-12-30" = { /* toolchain */ };
-      # ... other versions.
-    };
-
-    # ... Some internal attributes omitted.
+    "1.49.0" = { /* toolchain */ };
+    "1.48.0" = { /* toolchain */ };
+    # ... other versions.
   };
 
-  # These are for compatibility with nixpkgs-mozilla and
-  # provide same toolchains as `rust-bin.*`.
-  latest.rustChannels = /* ... */;
-  rustChannelOf = /* ... */;
-  rustChannelOfTargets = /* ... */;
-  rustChannels = /* ... */;
+  beta = {
+    # The latest beta toolchain.
+    latest = { /* toolchain */ };
+    "2021-01-01" = { /* toolchain */ };
+    "2020-12-30" = { /* toolchain */ };
+    # ... other versions.
+  };
+
+  nightly = {
+    # The latest nightly toolchain.
+    # It is preferred to use `selectLatestNightlyWith` instead of this since
+    # nightly toolchain may have components (like `rustfmt` or `rls`) missing,
+    # making `default` profile unusable.
+    latest = { /* toolchain */ };
+    "2020-12-31" = { /* toolchain */ };
+    "2020-12-30" = { /* toolchain */ };
+    # ... other versions.
+  };
+
+  # ... Some internal attributes omitted.
+}
+```
+
+## Flake output structure
+
+```nix
+{
+  lib.mkRustBin = { distRoot ? /*...*/ }: pkgs: «attrset rust-bin»;
+  overlays = {
+    # The overlay.
+    default = final: prev: «attrset»;
+    # Alias to `default`.
+    rust-overlay = final: prev: «attrset»;
+  };
+
+  # WARNING: The structure of `packages` here is to-be-determined and may be
+  # renamed or modified in the future.
+  packages.${system} = {
+    # Stable toolchain of release major.minor.patch
+    # The derivation is the "default" profile, and the "minimal" profile can be
+    # accessed via `minimal` attribute.
+    # This type is abbreviated as `«toolchain»` below.
+    "rust_${major}_${minor}_${patch}" = «derivation» // { minimal = «derivation»; };
+    # Alias to `rust_${latest-major-minor-patch}`.
+    rust = «toolchain»; 
+    # Alias to `rust`.
+    default = «toolchain»; 
+
+    # Nightly toolchain of a specific date.
+    "rust-nightly_${yyyy}-${mm}-${dd}" = «toolchain»;
+    # Alias to `rust-nightly_${latest-nightly-yyyy-mm-dd}`.
+    rust-nightly = «toolchain»; 
+
+    # Beta toolchain of a specific date.
+    "rust-beta_${yyyy}-${mm}-${dd}" = «toolchain»;
+    # Alias to `rust-beta_${latest-beta-yyyy-mm-dd}`.
+    rust-beta = «toolchain»; 
+  };
+
+  # ... Some internal attributes omitted.
 }
 ```