pkg/eval/vals: Print floats with exponents between -4 and 13 with %f.

This fixes #811.

pkg/eval/vals/string.go

@@ -1,6 +1,9 @@
 package vals
 
-import "strconv"
+import (
+	"strconv"
+	"strings"
+)
 
 // Stringer wraps the String method.
 type Stringer interface {
@@ -25,5 +28,18 @@ func ToString(v interface{}) string {
 }
 
 func formatFloat64(f float64) string {
-	return strconv.FormatFloat(f, 'g', -1, 64)
+	// Go's 'g' format is almost what we want, except that its threshold for
+	// "large exponent" is too low - 6 for positive exponents and -5 for
+	// negative exponents. This means that relative small numbers like 1234567
+	// are printed with scientific notations, something we don't really want.
+	// See also b.elv.sh/811.
+	//
+	// So we emulate the 'g' format by first using 'f', parse the result, and
+	// use 'e' if the exponent >= 14 or <= -5.
+	s := strconv.FormatFloat(f, 'f', -1, 64)
+	i := strings.IndexByte(s, '.')
+	if (i == -1 && len(s) > 14) || i >= 14 || strings.HasPrefix(s, "0.0000") {
+		return strconv.FormatFloat(f, 'e', -1, 64)
+	}
+	return s
 }

pkg/eval/vals/string_test.go

@@ -11,9 +11,17 @@ func TestToString(t *testing.T) {
 	Test(t, Fn("ToString", ToString), Table{
 		// string
 		Args("a").Rets("a"),
+
 		// float64
 		Args(42.0).Rets("42"),
 		Args(0.1).Rets("0.1"),
+		// Exponents >= 14 are printed in scientific notation.
+		Args(1e13).Rets("10000000000000"),
+		Args(1e14).Rets("1e+14"),
+		// Exponents <= -5 are printed in scientific notation.
+		Args(1e-4).Rets("0.0001"),
+		Args(1e-5).Rets("1e-05"),
+
 		// Stringer
 		Args(bytes.NewBufferString("buffer")).Rets("buffer"),
 		// None of the above: delegate to Repr