Convert error types into productive exit codes

README.md

@@ -181,6 +181,22 @@ Language selection is based on precedence, in the following order:
 * Detected from the input file's extension
 * A specified query file
 
+#### Exit Codes
+
+The Topiary process will exit with a zero exit code upon successful
+formatting. Otherwise, the following exit codes are defined:
+
+| Reason                       | Code |
+| :--------------------------- | ---- |
+| Unspecified error            |    1 |
+| CLI argument parsing error   |    2 |
+| I/O error                    |    3 |
+| Topiary query error          |    4 |
+| Source parsing error         |    5 |
+| Language detection error     |    6 |
+| Idempotency error            |    7 |
+| Unspecified formatting error |    8 |
+
 #### Example
 
 Once built, the program can be run like this:

flake.nix

@@ -9,7 +9,7 @@
       "tweag-topiary.cachix.org-1:8TKqya43LAfj4qNHnljLpuBnxAY/YwEBfzo3kzXxNY0="
     ];
   };
-  
+
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
@@ -34,7 +34,8 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         code = pkgs.callPackage ./. { inherit nixpkgs system advisory-db crane rust-overlay nix-filter; };
-      in {
+      in
+      {
         packages = with code; {
           inherit web-playground;
           default = app;
@@ -48,17 +49,29 @@
           enable = true;
           name = "topiary";
           description = "A general code formatter based on tree-sitter.";
-          entry = let
-            topiary-inplace = pkgs.writeShellApplication {
-              name = "topiary-inplace";
-              text = ''
-                for file; do
-                  ${code.app}/bin/topiary --in-place --input-file "$file"
-                done
-              '';
-            };
-          in "${topiary-inplace}/bin/topiary-inplace";
-          files = "(\\.json$)|(\\.toml$)|(\\.mli?$)";
+          entry =
+            let
+              topiary-inplace = pkgs.writeShellApplication {
+                name = "topiary-inplace";
+                text = ''
+                  for FILE; do
+                    if ${code.app}/bin/topiary --in-place --input-file "$FILE"; then
+                      continue
+                    else
+                      EXIT=$?
+                      if (( EXIT == 6 )); then
+                        # Skip over language detection errors
+                        continue
+                      else
+                        exit $EXIT
+                      fi
+                    fi
+                  done
+                '';
+              };
+            in
+            "${topiary-inplace}/bin/topiary-inplace";
+          types = [ "text" ];
         };
       }
     );

src/bin/topiary/error.rs

@@ -1,4 +1,4 @@
-use std::{error, fmt, io, result};
+use std::{error, fmt, io, process::ExitCode, result};
 use topiary::FormatterError;
 
 /// A convenience wrapper around `std::result::Result<T, TopiaryError>`.
@@ -40,6 +40,39 @@ impl error::Error for TopiaryError {
     }
 }
 
+impl From<TopiaryError> for ExitCode {
+    fn from(e: TopiaryError) -> Self {
+        let exit_code = match e {
+            // Formatting errors: Exit 8
+            TopiaryError::Lib(FormatterError::Formatting(_)) => 8,
+
+            // Idempotency errors: Exit 7
+            TopiaryError::Lib(FormatterError::Idempotence) => 7,
+
+            // Language detection errors: Exit 6
+            TopiaryError::Lib(FormatterError::LanguageDetection(_, _)) => 6,
+
+            // Parsing errors: Exit 5
+            TopiaryError::Lib(FormatterError::Parsing { .. }) => 5,
+
+            // Query errors: Exit 4
+            TopiaryError::Lib(FormatterError::Query(_, _)) => 4,
+
+            // I/O errors: Exit 3
+            TopiaryError::Lib(FormatterError::Io(_)) => 3,
+            TopiaryError::Bin(_, Some(CLIError::IOError(_))) => 3,
+
+            // Bad arguments: Exit 2
+            // (Handled by clap: https://github.com/clap-rs/clap/issues/3426)
+
+            // Anything else: Exit 1
+            _ => 1,
+        };
+
+        ExitCode::from(exit_code)
+    }
+}
+
 impl From<FormatterError> for TopiaryError {
     fn from(e: FormatterError) -> Self {
         Self::Lib(e)

src/bin/topiary/main.rs

@@ -8,6 +8,7 @@ use std::{
     fs::File,
     io::{stdin, BufReader, BufWriter, Read},
     path::PathBuf,
+    process::ExitCode,
 };
 
 use clap::{ArgGroup, Parser};
@@ -68,11 +69,13 @@ struct Args {
 }
 
 #[tokio::main]
-async fn main() {
+async fn main() -> ExitCode {
     if let Err(e) = run().await {
         print_error(&e);
-        std::process::exit(1);
+        return e.into();
     }
+
+    ExitCode::SUCCESS
 }
 
 async fn run() -> CLIResult<()> {