internal/core/convert: allow results of math to be integer

Change-Id: I1f8c4c2ce597335e8f8809c38ce2be790ba7d592
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7882
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/format/printer.go

@@ -137,6 +137,11 @@ func (p *printer) Print(v interface{}) {
 			if p := strings.IndexByte(data, '.'); p >= 0 && data[p+1] > '9' {
 				data = data[:p+1] + "0" + data[p+1:]
 			}
+			// Lowercase E, but only if it is not the last character: in the
+			// future we may use E for Exa.
+			if p := strings.IndexByte(data, 'E'); p != -1 && p < len(data)-1 {
+				data = strings.ToLower(data)
+			}
 
 		case token.FLOAT:
 			// Pad leading or trailing dots.

internal/core/convert/go.go

@@ -284,10 +284,13 @@ func convertRec(ctx *adt.OpContext, nilIsTop bool, x interface{}) adt.Value {
 		// a float or an int after all.
 		// The code to autodetect whether something is an integer can be done
 		// with this:
-		// var d apd.Decimal
-		// res, _ := apd.BaseContext.RoundToIntegralExact(&d, v)
-		// integer := !res.Inexact()
-		n := &adt.Num{Src: ctx.Source(), K: adt.FloatKind}
+		kind := adt.FloatKind
+		var d apd.Decimal
+		res, _ := apd.BaseContext.RoundToIntegralExact(&d, v)
+		if !res.Inexact() {
+			kind = adt.IntKind
+		}
+		n := &adt.Num{Src: ctx.Source(), K: kind}
 		n.X = *v
 		return n