Drop the CLI's no-data hint, keep the number underflow rule in grammar/number.go, and name the JSON a template may be

README.md

@@ -30,7 +30,7 @@ born {date(1990-01-01,2010-12-31,'2006-01-02')}
 EOF
 ```
 
-fejkdata renders the template on its stdin: a format string, or any JSON value. Its `{…}` tokens reach the data by reference from the root,
+fejkdata renders the template on its stdin: 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}`, so shipped and `--data-path` categories are alike available,
 and everything outside them prints as written. A reference names a category, or a field
 inside one: each dot segment descends one level — folders, then the category (a JSON

cmd/fejkdata/main.go

@@ -35,7 +35,7 @@ fejkdata <<'EOF'
 born {date(1990-01-01,2010-12-31,'2006-01-02')}
 EOF
 
-A template is a format string, or any JSON value. Its {…} tokens
+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
@@ -606,7 +606,7 @@ func run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 		return 0
 	}
 	if err := in.write(f, src, stdout); err != nil {
-		return fail(stderr, in.noDataCause(err))
+		return fail(stderr, err)
 	}
 	return 0
 }
@@ -622,18 +622,6 @@ func readStdin(stdin io.Reader) (input, error) {
 	return parseInput(string(raw)), nil
 }
 
-// noDataCause adds the flags that left the generator without data to an error saying none is
-// loaded.
-func (in invocation) noDataCause(err error) error {
-	if !in.noShipped || len(in.dirs) > 0 || !strings.HasSuffix(err.Error(), "no data is loaded") {
-		return err
-	}
-	if te, isInput := err.(inputError); isInput {
-		return inputError{fmt.Errorf("%w: --no-shipped-data and no --data-path", te.error)}
-	}
-	return fmt.Errorf("%w: --no-shipped-data and no --data-path", err)
-}
-
 func fail(stderr io.Writer, err error) int {
 	var te inputError
 	if errors.As(err, &te) {

internal/grammar/calc.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"slices"
 	"strconv"
-	"strings"
 	"unicode"
 )
 
@@ -178,10 +177,3 @@ func (p *calcParser) ident() (CalcNode, error) {
 	}
 	return CalcVar{name, at}, nil
 }
-
-// Underflows reports a number written as text that reads as f = 0 though a digit before its
-// exponent is not 0, as 1e-400 does: no float64 holds the number written.
-func Underflows(text string, f float64) bool {
-	mantissa, _, _ := strings.Cut(strings.ToLower(text), "e")
-	return f == 0 && strings.ContainsAny(mantissa, "123456789")
-}

internal/grammar/number.go

@@ -0,0 +1,10 @@
+package grammar
+
+import "strings"
+
+// Underflows reports a number written as text that reads as f = 0 though a digit before its
+// exponent is not 0, as 1e-400 does: no float64 holds the number written.
+func Underflows(text string, f float64) bool {
+	mantissa, _, _ := strings.Cut(strings.ToLower(text), "e")
+	return f == 0 && strings.ContainsAny(mantissa, "123456789")
+}