Add a lot of documentation to the Flow library

flow/readme.md

@@ -0,0 +1,3 @@
+# Flow
+
+This is a library to help with application control flow, separating fatal errors from normal recoverable errors. This is almost entirely inspired by [Error Handling in a Correctness-Critical Rust Project](https://sled.rs/errors.html) in the sled.rs library.

flow/src/lib.rs

@@ -1,14 +1,40 @@
+//! Control Flow for Correctness-Critical applications
+//!
+//! https://sled.rs/errors.html
+//!
+//! Where the sled.rs library uses `Result<Result<A, Error>, FatalError>`, these are a little hard to
+//! work with. This library works out a set of utility functions that allow us to work with the
+//! nested errors in the same way as a regular Result.
 use std::error::Error;
 
+/// Implement this trait for the application's fatal errors.
+///
+/// Fatal errors should be things that should trigger an application shutdown. Applications should
+/// not try to recover from fatal errors, but simply bring the app to the safest shutdown point and
+/// report the best possible information to the user.
+///
+/// Examples: database corruption, or the database is unavailable in an application that cannot
+/// function without it. Graphics environment cannot be initialized in a GUI app.
+///
+/// Applications should generally have only one FatalError type. There are no handling utilities
+/// for Fatal conditions, so Fatal conditions must be handled through an ordinary `match`
+/// statement.
 pub trait FatalError: Error {}
 
+/// Flow<A, FE, E> represents a return value that might be a success, might be a fatal error, or
+/// might be a normal handleable error.
 pub enum Flow<A, FE, E> {
+    /// The operation was successful
     Ok(A),
+    /// The operation encountered a fatal error. These should be bubbled up to a level that can
+    /// safely shut the application down.
     Fatal(FE),
+    /// Ordinary errors. These should be handled and the application should recover gracefully.
     Err(E),
 }
 
 impl<A, FE, E> Flow<A, FE, E> {
+    /// Apply an infallible function to a successful value.
     pub fn map<B, O>(self, mapper: O) -> Flow<B, FE, E>
     where
         O: FnOnce(A) -> B,
@@ -20,6 +46,9 @@ impl<A, FE, E> Flow<A, FE, E> {
         }
     }
 
+    /// Apply a potentially fallible function to a successful value.
+    ///
+    /// Like `Result.and_then`, the mapping function can itself fail.
     pub fn and_then<B, O>(self, handler: O) -> Flow<B, FE, E>
     where
         O: FnOnce(A) -> Flow<B, FE, E>,
@@ -31,6 +60,9 @@ impl<A, FE, E> Flow<A, FE, E> {
         }
     }
 
+    /// Map a normal error from one type to another. This is useful for converting an error from
+    /// one type to another, especially in re-throwing an underlying error. `?` syntax does not
+    /// work with `Flow`, so you will likely need to use this a lot.
     pub fn map_err<F, O>(self, mapper: O) -> Flow<A, FE, F>
     where
         O: FnOnce(E) -> F,
@@ -42,6 +74,7 @@ impl<A, FE, E> Flow<A, FE, E> {
         }
     }
 
+    /// Provide a function to use to recover from (or simply re-throw) an error.
     pub fn or_else<O, F>(self, handler: O) -> Flow<A, FE, F>
     where
         O: FnOnce(E) -> Flow<A, FE, F>,
@@ -54,6 +87,8 @@ impl<A, FE, E> Flow<A, FE, E> {
     }
 }
 
+/// Convert from a normal `Result` type to a `Flow` type. The error condition for a `Result` will
+/// be treated as `Flow::Err`, never `Flow::Fatal`.
 impl<A, FE, E> From<Result<A, E>> for Flow<A, FE, E> {
     fn from(r: Result<A, E>) -> Self {
         match r {
@@ -63,19 +98,23 @@ impl<A, FE, E> From<Result<A, E>> for Flow<A, FE, E> {
     }
 }
 
+/// Convenience function to create an ok value.
 pub fn ok<A, FE: FatalError, E: Error>(val: A) -> Flow<A, FE, E> {
     Flow::Ok(val)
 }
 
+/// Convenience function to create an error value.
 pub fn error<A, FE: FatalError, E: Error>(err: E) -> Flow<A, FE, E> {
     Flow::Err(err)
 }
 
+/// Convenience function to create a fatal value.
 pub fn fatal<A, FE: FatalError, E: Error>(err: FE) -> Flow<A, FE, E> {
     Flow::Fatal(err)
 }
 
 #[macro_export]
+/// Return early from the current function if the value is a fatal error.
 macro_rules! return_fatal {
     ($x:expr) => {
         match $x {
@@ -87,6 +126,7 @@ macro_rules! return_fatal {
 }
 
 #[macro_export]
+/// Return early from the current function is the value is an error.
 macro_rules! return_error {
     ($x:expr) => {
         match $x {