Should AppliedFragment Monoid use spaces?

[matthughes]

When combining AppliedFragments, should we intercalcate a space? I think technically that would violate the Monoid rules but I think that is what callers would want 99% of the time.

  /** `AppliedFragment` is a monoid. */
  implicit val MonoidAppFragment: Monoid[AppliedFragment] =
    new Monoid[AppliedFragment] {
      def combine(x: AppliedFragment, y: AppliedFragment): AppliedFragment = x |+| y
      def empty: AppliedFragment = AppliedFragment.empty
    }

[mpilquist]

I think what you're really after is for Fragment#sql to insert spaces between parts. That is:

diff --git a/modules/core/shared/src/main/scala/Fragment.scala b/modules/core/shared/src/main/scala/Fragment.scala
index 26bfc4d5..1502fa03 100644
--- a/modules/core/shared/src/main/scala/Fragment.scala
+++ b/modules/core/shared/src/main/scala/Fragment.scala
@@ -26,7 +26,7 @@ final case class Fragment[A](
     parts.traverse {
       case Left(s)  => s.pure[State[Int, *]]
       case Right(s) => s
-    } .runA(1).value.combineAll
+    }.runA(1).value.intercalate(" ")

   def query[B](decoder: Decoder[B]): Query[A, B] =
     Query(sql, origin, encoder, decoder, isDynamic = false)

Doing so causes 10 test failures though, including left/right identity tests on AppliedFragment.

Can you describe your use case a bit more? Maybe there's some higher level utility we can provide to better address the use case without changing the internals of fragment composition.