Use - to denote reading from stdin

Instead of taking --stdin-format to imply reading from stdin, use the
convention of supplying `-` as an argument to denote reading from
stdin, and just use the flag to alter how it's read.

This will line up better with expectations; and, since --stdin-format
can have a default (of "yaml"), invocations can be concise:

    jk transform ./script.js -

.. rather than

    jk transform ./script.js --stdin-format=yaml

Note that _no_ arguments does not imply reading from stdin. I did not
want something that accidentally supplies no filenames to block, since
that would make automation using `jk transform` brittle.

std/cmd/transform.ts

@@ -29,15 +29,16 @@ function transform(fn: TransformFn): void {
   const inputFiles = param.Object('jk.transform.input', {});
   const outputs = [];
 
-  const stdinFormat = param.String('jk.transform.stdin.format', '');
-  if (stdinFormat !== '') {
-    const format = valuesFormatFromExtension(stdinFormat);
-    const path = `stdin.${stdinFormat}`;  // path is a stand-in
-    const value = read(stdin, { format }).then(v => v.map(transformOne));
-    outputs.push({ path, value, format });
-  }
-
   for (const path of Object.keys(inputFiles)) {
+    if (path === '') { // read from stdin
+      const stdinFormat = param.String('jk.transform.stdin.format', 'yaml');
+      const format = valuesFormatFromExtension(stdinFormat);
+      const path = `stdin.${stdinFormat}`;  // path is a stand-in
+      const value = read(stdin, { format }).then(v => v.map(transformOne));
+      outputs.push({ path, value, format });
+      continue;
+    }
+
     const format = valuesFormatFromPath(path);
     outputs.push(host.read(path, { format }).then((obj): File => {
       switch (format) {

transform.go

@@ -18,9 +18,19 @@ var transformCmd = &cobra.Command{
 }
 
 const transformExamples = `
-  running the default export of a module (or file) on each input document
+
+  run the default export of script.js on each YAML value read from
+  stdin, printing the transformed values to stdout
+
+    jk transform ./script.js -
+
+  run the default export of a module (or file) on each input document,
+  and write the results back to outputdir/
+
     jk transform -o outputdir/ ./script.js ./inputdir/*.json
-  running a function on each input, and printing the results to stdout
+
+  run a function on each input file, and print the results to stdout
+
     jk transform --stdout -c '({ name: n, ...fields }) => ({ name: n + "-dev", ...fields })' inputdir/*.yaml
 `
 
@@ -33,17 +43,18 @@ var transformOptions struct {
 	overwrite bool
 	// force the format of the output
 	format string
-	// read from stdin, expecting a stream of values in the format given
+	// when reading from stdin (`-` is supplied as an argument),
+	// expect a stream of values in the format given
 	stdinFormat string
 }
 
 func init() {
 	initScriptFlags(transformCmd, &transformOptions.scriptOptions)
 	initExecFlags(transformCmd, &transformOptions.vmOptions)
-	transformCmd.PersistentFlags().BoolVar(&transformOptions.stdout, "stdout", false, "print the resulting values to stdout")
+	transformCmd.PersistentFlags().BoolVar(&transformOptions.stdout, "stdout", false, "print the resulting values to stdout (implied if reading from stdin)")
 	transformCmd.PersistentFlags().BoolVar(&transformOptions.overwrite, "overwrite", false, "allow input file(s) to be overwritten by output file(s); otherwise, an error will be thrown")
-	transformCmd.PersistentFlags().StringVar(&transformOptions.format, "format", "", "force all values to this format")
-	transformCmd.PersistentFlags().StringVar(&transformOptions.stdinFormat, "stdin-format", "", "read values from stdin, assuming this format; implies --stdout")
+	transformCmd.PersistentFlags().StringVar(&transformOptions.format, "format", "", "force all output vlaues to this format")
+	transformCmd.PersistentFlags().StringVar(&transformOptions.stdinFormat, "stdin-format", "yaml", "assume this format for values from stdin (either a 'yaml' stream or concatenated 'json' values)")
 	jk.AddCommand(transformCmd)
 }
 
@@ -64,11 +75,18 @@ func transform(cmd *cobra.Command, args []string) {
 	// now). This is in part to get around the limitations of
 	// parameters (arrays are not supported as values), and partly in
 	// anticipation of there being more information to pass about each
-	// input.
+	// input. Stdin is encoded as an empty string, since `"-"` could
+	// be the name of a file.
 	inputs := make(map[string]interface{})
 	for _, f := range args[1:] {
-		inputs[f] = f
+		if f == "-" {
+			inputs[""] = ""
+			transformOptions.stdout = true
+		} else {
+			inputs[f] = f
+		}
 	}
+
 	vm.parameters.Set("jk.transform.input", inputs)
 	vm.parameters.Set("jk.transform.stdout", transformOptions.stdout)
 	vm.parameters.Set("jk.transform.overwrite", transformOptions.overwrite)