Extended Try RFC

Author: TyOverby

active/0000-extended-try.md

@@ -0,0 +1,71 @@
+- Start Date: 2014-06-25
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Extend the `try!` macro to include an optional second parameter that is a constructor to wrap
+around the original error in case of failure.
+
+# Motivation
+
+`try!` is a useful macro when dealing with many functions that return `Result`s, but they become
+useless when the `Result` type that the programmer wants to return has a different failure type.
+For example, in a function that uses Io and Regex, two different error types could be generated
+(IoError, and Regex::Error).  The author could not choose either of these errors to return because
+neither is extendable with the other.  Instead it is common for library and application authors
+to create their own error types that wrap the errors that could possibly be produced.  Unfortunately,
+this means that the `try!` macro is no longer useful.
+
+# Detailed design
+
+This RFC proposes adding another argument to the `try!` macro that would be used as a constructor
+to wrap around existing error types.  For example:
+
+```rust
+enum MyError {
+  IoFailed(IoError),
+  RegexFailed(regex::Error)
+}
+
+fn read_then_regex(filename: &str, regex: &str) -> MyError {
+   let file = try!(File::open(filename), IoFailed);
+   let regex = try!(Regex::new(regex), RegexFailed);
+   // do things
+}
+
+```
+
+The desugared version of this example (which is required to implement this pattern today)
+would look like:
+
+```rust
+fn read_then_regex(filename: &str, regex: &str) -> MyError {
+   let file = match File::open(filename) {
+     Ok(a) => a,
+     Err(x) => IoFailed(x)
+   };
+   let regex = match Regex::new(regex) {
+     Ok(a) => a,
+     Err(x) => RegexFailed(x)
+   };
+   // do things
+}
+```
+
+The motivation for this improvement is the exact same as the motivation for the original `try!`
+macro.
+
+The original form of the `try!` macro would still be valid and would continue to work without
+any changes.
+
+# Drawbacks
+
+Adds confusion.  It is not immediately obvious as to what the 2nd argument is for if
+the reader is not already familiar with it.
+
+# Alternatives
+
+* Create another macro that is very similar
+  * (named `try_or!` ?).
+* Create another macro that is very similar and place it in an external library