Implement treefmt's stdin spec

This completes #305

Author: jfly

README.md

@@ -291,9 +291,9 @@ to return back to the unmerged state.
 
 ## Usage
 
-* `nixfmt < input.nix` – reads Nix code from `stdin`, formats it, and outputs to `stdout`
+* `echo "{a=1;}" | nixfmt --stdin-filepath input.nix` – reads Nix code from `stdin`, formats it, and outputs to `stdout` (the filepath (`input.nix`) is only used for error messages)
 * `nixfmt file.nix` – format the file in place
 
-## Acknowledgements
+## Acknowledgments
 
 `nixfmt` was originally developed by [Serokell](https://github.com/serokell) and later donated to become an official Nix project with the acceptance of [RFC 166](https://github.com/NixOS/rfcs/pull/166).

main/Main.hs

@@ -15,7 +15,6 @@ import Data.ByteString.Char8 (unpack)
 import Data.Either (lefts)
 import Data.FileEmbed
 import Data.List (intersperse, isSuffixOf)
-import Data.Maybe (fromMaybe)
 import Data.Text (Text)
 import qualified Data.Text.IO as TextIO (getContents, hPutStr, putStr)
 import Data.Version (showVersion)
@@ -57,7 +56,7 @@ data Nixfmt = Nixfmt
     strict :: Bool,
     verify :: Bool,
     ast :: Bool,
-    filename :: Maybe FilePath,
+    stdin_filepath :: Maybe FilePath,
     ir :: Bool
   }
   deriving (Show, Data, Typeable)
@@ -89,11 +88,11 @@ options =
           False
             &= help
               "Pretty print the internal AST, only for debugging",
-        filename =
+        stdin_filepath =
           Nothing
             &= help
-              "The filename to display when the file input is given through stdin.\n\
-              \Useful for tools like editors and autoformatters that wish to use Nixfmt without providing it direct file access, while still providing context to where the file is.",
+              "Format the contents of stdin. When specified, no positional arguments are allowed.\n\
+              \The filepath is is only used for error messages.",
         ir =
           False
             &= help
@@ -153,8 +152,8 @@ checkTarget format Target{tDoRead, tPath} = do
       | formatted == contents -> Right ()
       | otherwise -> Left $ tPath ++ ": not formatted"
 
-stdioTarget :: Maybe FilePath -> Target
-stdioTarget filename = Target TextIO.getContents (fromMaybe "<stdin>" filename) (const TextIO.putStr)
+stdioTarget :: FilePath -> Target
+stdioTarget filepath = Target TextIO.getContents filepath (const TextIO.putStr)
 
 fileTarget :: FilePath -> Target
 fileTarget path = Target (readFileUtf8 path) path atomicWriteFile
@@ -169,8 +168,8 @@ checkFileTarget :: FilePath -> Target
 checkFileTarget path = Target (readFileUtf8 path) path (const $ const $ pure ())
 
 toTargets :: Nixfmt -> IO [Target]
-toTargets Nixfmt{files = [], filename} = pure [stdioTarget filename]
-toTargets Nixfmt{files = ["-"], filename} = pure [stdioTarget filename]
+toTargets Nixfmt{files = [], stdin_filepath = Just filepath} = pure [stdioTarget filepath]
+toTargets Nixfmt{files = _ : _, stdin_filepath = Just _} = error "Cannot specify both positional files and --stdin-filepath"
 toTargets Nixfmt{check = False, files = paths} = map fileTarget <$> collectAllNixFiles paths
 toTargets Nixfmt{check = True, files = paths} = map checkFileTarget <$> collectAllNixFiles paths
 

test/test.sh

@@ -1,5 +1,10 @@
 #!/usr/bin/env bash
 
+# We invoke `nixmft` in a way that makes shellcheck think we're clobbering the
+# file we're formatting as we're reading it. That's not actually happening
+# here.
+# shellcheck disable=SC2094
+
 set -euo pipefail
 
 # Simple test runner for nixfmt.
@@ -35,7 +40,7 @@ verifyCorrect() {
   shift
 
   echo "Checking $file (${*:-no options}) …"
-  if ! out=$(nixfmt --strict --verify "$@" < "$file"); then
+  if ! out=$(nixfmt --strict --verify --stdin-filepath "$file" "$@" < "$file"); then
     echo "[ERROR] failed nixfmt verification"
     exit 1
   fi
@@ -60,7 +65,7 @@ verifyCorrect test/correct-indent-4.nix --indent=4
 
 # Verify "invalid"
 for file in test/invalid/*.nix; do
-  if nixfmt < "$file" > /dev/null 2>&1; then
+  if nixfmt --stdin-filepath "test-$file" < "$file" > /dev/null 2>&1; then
     echo "[ERROR] $file should have failed nixfmt"
     exit 1
   else
@@ -71,7 +76,7 @@ done
 # Verify "diff"
 for file in test/diff/**/in.nix; do
   echo "Checking $file …"
-  out="$(nixfmt --verify < "$file")"
+  out="$(nixfmt --verify --stdin-filepath "$file" < "$file")"
   outfile="$(dirname "$file")/out.nix"
 
   if diff --color=always --unified "$outfile" <(echo "$out"); then
@@ -85,7 +90,7 @@ for file in test/diff/**/in.nix; do
   fi
 
   echo "Checking $file with --strict …"
-  out="$(nixfmt --strict --verify < "$file")"
+  out="$(nixfmt --strict --verify  --stdin-filepath "$file" < "$file")"
   outfile="$(dirname "$file")/out-pure.nix"
 
   if diff --color=always --unified "$outfile" <(echo "$out"); then