Give the help examples that read no data and the exit codes, say a data path is no directory in one shape, suggest a record of one for a choice, name a JSON syntax error's line and column, and say no data is loaded from FakeStruct

cli/cli.go

@@ -27,8 +27,9 @@ const usage = `Usage: <template> | COMMAND [flags]
 
 COMMAND reads one template from stdin and renders it:
 
-  echo 'name: {/sv_SE.person.last}' | COMMAND
+  echo 'id: {int(1,999)}' | COMMAND
   echo '{"format":"{x}","x":["bosse","lina"]}' | COMMAND
+  COMMAND --list                     # the paths a {/path} reference reads
   COMMAND < template.txt
 
 COMMAND <<'EOF'
@@ -36,11 +37,10 @@ born {date(1990-01-01,2010-12-31,'2006-01-02')}
 EOF
 
 A template is a format string, or a JSON object, array, string or null. Its {…} tokens
-reach the data by reference from the root, {/sv_SE.person.last}, whether the data
-is shipped or layered with --data-path. A reference names a category or a dotted
-path into one ({/sv_SE.person.last}), a table's row by key or name
-({/misc.territory[SE]}), and .. steps up from a row to the row it links to
-({/geo.SE.locality..municipality.name}).
+reach the data by reference from the root, {/path}, for any path --list prints, whether
+the data is shipped or layered with --data-path. A reference names a category or a
+dotted path into one, a table's row by key or name ({/table[KEY]}), and .. steps up
+from a row to the row it links to.
 
 Under --format text, the default, what you send is what renders, and COMMAND adds
 nothing: a format string keeps every byte, so echo's newline ends each render, and
@@ -51,7 +51,7 @@ format string like any other, and renders as sent. A quoted heredoc, <<'EOF', pa
 a backslash or a quote as written.
 
 With --format json, ndjson, csv or sql the template must be a record — a JSON
-template whose fields are its columns, or a lone reference such as {/sv_SE.person}:
+template whose fields are its columns, or a lone reference such as {/path}:
 one reference and nothing else, with one newline after it allowed — and the rows are
 written as one JSON array, one JSON object per line, one CSV row (after a header), or
 one INSERT.
@@ -73,6 +73,8 @@ one INSERT.
 
 A short flag's value attaches or follows (-n3, -n 3); short flags bundle (-hn 3);
 -- ends the flags.
+
+Exit codes: 0 success, 1 runtime error, 2 misuse.
 `
 
 type invocation struct {
@@ -279,7 +281,7 @@ func (in invocation) checkFlags() error {
 	return nil
 }
 
-func pipeHint(name string) string { return "echo '{/sv_SE.person}' | " + name }
+func pipeHint(name string) string { return "echo 'id: {int(1,999)}' | " + name }
 
 // operandError refuses an argument that is no flag, naming the spelling that reads what
 // it most likely means: dropping it for -, a file for a file's name, else a template

internal/datafiles/datafiles.go

@@ -2,6 +2,7 @@
 package datafiles
 
 import (
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -91,15 +92,25 @@ func (s Source) check() error {
 		return fmt.Errorf("a data path is empty")
 	}
 	info, err := os.Stat(s.label)
+	if errors.Is(err, fs.ErrNotExist) {
+		return noDirectory(s.label)
+	}
 	if err != nil {
 		return fmt.Errorf("data path %s: %w", s.label, err)
 	}
 	if !info.IsDir() {
-		return fmt.Errorf("%s is not a directory", s.label)
+		return fmt.Errorf("data path %s: not a directory", s.label)
 	}
 	return nil
 }
 
+// noDirectory is a data path naming nothing, which errors.Is still matches as fs.ErrNotExist.
+type noDirectory string
+
+func (d noDirectory) Error() string { return "data path " + string(d) + ": no such directory" }
+
+func (noDirectory) Unwrap() error { return fs.ErrNotExist }
+
 // walkDir walks the folder that dir names, and reports whether it handed over any category.
 func (s Source) walkDir(dir []string, compile func(Category) error) (bool, error) {
 	full := s.dirPath(dir)

internal/jsonvalue/jsonvalue.go

@@ -4,7 +4,9 @@ package jsonvalue
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"unicode/utf8"
 )
 
 // Value is a JSON value that only Decode, String and Object make, so a template never compiles
@@ -15,7 +17,7 @@ type Value struct{ v any }
 func Decode(b []byte) (Value, error) {
 	var v any
 	if !json.Valid(b) {
-		return Value{}, json.Unmarshal(b, &v)
+		return Value{}, located(b, json.Unmarshal(b, &v))
 	}
 	dec := json.NewDecoder(bytes.NewReader(b))
 	dec.UseNumber()
@@ -23,6 +25,18 @@ func Decode(b []byte) (Value, error) {
 	return Value{v}, err
 }
 
+// located prefixes a syntax error with the line and column of the byte it stopped at.
+func located(b []byte, err error) error {
+	var syntax *json.SyntaxError
+	if !errors.As(err, &syntax) {
+		return err
+	}
+	before := b[:max(syntax.Offset-1, 0)]
+	line := bytes.Count(before, []byte("\n")) + 1
+	column := utf8.RuneCount(before[bytes.LastIndexByte(before, '\n')+1:]) + 1
+	return fmt.Errorf("line %d, column %d: %w", line, column, err)
+}
+
 // String is the JSON string s.
 func String(s string) Value { return Value{s} }
 

record.go

@@ -143,7 +143,7 @@ func (f *Generator) FakeRecord(path string) (*Record, error) {
 		return nil, fmt.Errorf("fejkdata: %s descends into %q, a field; only a category-level template is a record", path, tail[0])
 	}
 	record, err := recordOf(n)
-	if errors.Is(err, ErrNoColumns) {
+	if errors.Is(err, ErrNoColumns) || errors.Is(err, errChoiceRecord) {
 		ns := grammar.IdentifierSegments(segments)
 		return nil, fmt.Errorf(`fejkdata: %s %w; render it as a column of one: {"format":"","%s":"{/%s}"}`, path, err, ns[len(ns)-1], path)
 	}
@@ -209,10 +209,12 @@ func (f *Generator) FakeRecordTemplate(input string) (*Record, error) {
 	return t.Fake(), nil
 }
 
-// ErrNoColumns is the one record fence a path can answer, so its entry point names
-// the record to write instead.
+// ErrNoColumns and errChoiceRecord are the record fences a path can answer, so its entry
+// point names the record to write instead.
 var ErrNoColumns = errors.New("has no fields, so no columns")
 
+var errChoiceRecord = errors.New("names a choice, not a template; a record is a template whose fields are its columns")
+
 // recordOf is the record template n is; settleRecords fixed its columns at load.
 func recordOf(n node) (*template, error) {
 	if tb, isTable := n.(*table); isTable {
@@ -220,7 +222,7 @@ func recordOf(n node) (*template, error) {
 	}
 	t, ok := n.(*template)
 	if !ok {
-		return nil, errors.New("names a choice, not a template; a record is a template whose fields are its columns")
+		return nil, errChoiceRecord
 	}
 	if len(t.fields) == 0 {
 		return nil, ErrNoColumns

struct.go

@@ -31,7 +31,7 @@ func (f *Generator) FakeStruct(v any) error {
 	defer f.mu.Unlock()
 	shape, err := f.structShapeOf(p.Elem().Type())
 	if err != nil {
-		return fmt.Errorf("fejkdata: %w", err)
+		return f.noDataNote(fmt.Errorf("fejkdata: %w", err))
 	}
 	shape.fill(f.drawState, p.Elem())
 	return nil