Introduce LiteralNullConditionalExpression()

upup/pkg/fi/cloudup/awstasks/vpc.go

@@ -296,7 +296,10 @@ func (_ *VPC) RenderTerraform(t *terraform.TerraformTarget, a, e, changes *VPC)
 		return err
 	}
 
-	if err := t.AddOutputVariable("vpc_ipv6_cidr_length", terraformWriter.LiteralFunctionExpression("local.vpc_ipv6_cidr_block == null ? null : tonumber", "regex(\".*/(\\\\d+)\", local.vpc_ipv6_cidr_block)[0]")); err != nil {
+	if err := t.AddOutputVariable("vpc_ipv6_cidr_length", terraformWriter.LiteralNullConditionalExpression(
+		terraformWriter.LiteralTokens("local", "vpc_ipv6_cidr_block"),
+		terraformWriter.LiteralFunctionExpression("tonumber", "regex(\".*/(\\\\d+)\", local.vpc_ipv6_cidr_block)[0]"),
+	)); err != nil {
 		return err
 	}
 

upup/pkg/fi/cloudup/terraformWriter/literal.go

@@ -77,6 +77,16 @@ func LiteralWithIndex(s string) *Literal {
 	}
 }
 
+// LiteralNullConditionalExpression constructs a Literal which returns `null`
+// if the supplied "nullable" expression is null, otherwise returns "value".
+// It is the caller's responsibility to ensure the supplied parameters do not use operators
+// with lower precedence than the conditional operator.
+func LiteralNullConditionalExpression(nullable, value *Literal) *Literal {
+	return &Literal{
+		String: fmt.Sprintf("%s == null ? null : %s", nullable.String, value.String),
+	}
+}
+
 // SortLiterals sorts a list of Literal, by key.  It does so in-place
 func SortLiterals(v []*Literal) {
 	sort.Slice(v, func(i, j int) bool {