refactor: make show implement one of the methods

builtin/autoloc.mbt

@@ -40,8 +40,8 @@ pub(all) type SourceLoc
 fn SourceLoc::repr(self : Self) -> String = "%loc_to_string"
 
 ///|
-pub impl Show for SourceLoc with output(self, logger) {
-  logger.write_string(self.repr())
+pub impl Show for SourceLoc with to_string(self) {
+  self.repr()
 }
 
 ///|

builtin/double.mbt

@@ -290,8 +290,8 @@ pub fn Double::to_string(self : Double) -> String {
 // }
 
 ///|
-pub impl Show for Double with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for Double with to_string(self) {
+  Double::to_string(self)
 }
 
 ///|

builtin/pkg.generated.mbti

@@ -1305,8 +1305,9 @@ pub impl Shl for UInt
 pub impl Shl for UInt16
 pub impl Shl for UInt64
 
+#must_implement_one(output, to_string)
 pub(open) trait Show {
-  output(Self, &Logger) -> Unit
+  output(Self, &Logger) -> Unit = _
   to_string(Self) -> String = _
 }
 pub impl Show for Unit

builtin/show.mbt

@@ -13,47 +13,47 @@
 // limitations under the License.
 
 ///|
-pub impl Show for Unit with output(_self, logger) {
-  logger.write_string("()")
+pub impl Show for Unit with to_string(_self) {
+  "()"
 }
 
 ///|
-pub impl Show for Bool with output(self, logger) {
+pub impl Show for Bool with to_string(self) {
   if self {
-    logger.write_string("true")
+    "true"
   } else {
-    logger.write_string("false")
+    "false"
   }
 }
 
 ///|
-pub impl Show for Int with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for Int with to_string(self) {
+  Int::to_string(self)
 }
 
 ///|
-pub impl Show for Int64 with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for Int64 with to_string(self) {
+  Int64::to_string(self)
 }
 
 ///|
-pub impl Show for UInt with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for UInt with to_string(self) {
+  UInt::to_string(self)
 }
 
 ///|
-pub impl Show for UInt64 with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for UInt64 with to_string(self) {
+  UInt64::to_string(self)
 }
 
 ///|
-pub impl Show for Byte with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for Byte with to_string(self) {
+  Byte::to_string(self)
 }
 
 ///|
-pub impl Show for UInt16 with output(self, logger) {
-  logger.write_string(self.to_string())
+pub impl Show for UInt16 with to_string(self) {
+  UInt16::to_string(self)
 }
 
 ///|

builtin/traits.mbt

@@ -123,22 +123,32 @@ impl Logger with write_char(self, value) {
 
 ///|
 /// Trait for types that can be converted to `String`
+#must_implement_one(output, to_string)
 pub(open) trait Show {
   // `output` is used for composition of aggregate structure.
   // `output` writes a string representation of `self` to a logger.
   // `output` should produce a valid MoonBit-syntax representation if possible.
   // For example, `Show::output` for `String` should be quoted
-  output(Self, &Logger) -> Unit
+  // By default `output` is implemented using `to_string`,
+  // but some types, such as `String` and `Char`, may override `output`,
+  // for different (escaped) behavior when composed into larger structures.
+  output(Self, &Logger) -> Unit = _
   // `to_string` should be used by end users of `Show`,
   // for printing, interpolation, etc. only, and should not be used for composition.
   // By default `to_string` is implemented using `output` and a buffer,
   // but some types, such as `String`, may override `to_string`,
-  // for different (unescaped) behavior when interpolated/printed directly
+  // for different (unescaped) behavior when interpolated/printed directly.
   to_string(Self) -> String = _
 }
 
 ///|
-/// Default implementation for `Show::to_string`, uses a `StringBuilder`
+/// Default implementation for `Show::output`, uses `Show::to_string`.
+impl Show with output(self, logger) {
+  logger.write_string(self.to_string())
+}
+
+///|
+/// Default implementation for `Show::to_string`, uses a `StringBuilder`.
 impl Show with to_string(self) {
   let logger = StringBuilder::new()
   self.output(logger)