chore: prepare for the restrictions on match types in SIP-56 (close #12)

Nested "type test extractors" like `IsSingleton` are not allowed
in the match types of SIP-56. Some of them were redundant and were
removed. The others were turned in explicit inner match types.

core/src/main/scala/skunk/tables/internal/typeops.scala

@@ -85,19 +85,17 @@ inline def findT[C <: Tuple, Label <: Singleton]: Find[C, Label] =
 /** Pick only `TypedColumns` with certain labels */
 type Pick[Columns <: Tuple, Labels <: Tuple] <: Tuple =
   Labels match
-    case EmptyTuple          => EmptyTuple
-    case IsSingleton[h] *: t => Find[Columns, h] *: Pick[Columns, t]
-
-inline def pickT[C <: Tuple, Labels <: Tuple]: Pick[C, Labels] =
-  inline erasedValue[Labels] match
-    case _: EmptyTuple            => EmptyTuple
-    case _: (IsSingleton[h] *: t) => findT[C, h] *: pickT[C, t]
+    case EmptyTuple =>
+      EmptyTuple
+    case h *: t =>
+      h match
+        case Singleton => Find[Columns, h & Singleton] *: Pick[Columns, t]
 
 type Reify[C <: Tuple] <: Tuple =
   C match
     case EmptyTuple =>
       EmptyTuple
-    case TypedColumn[IsSingleton[n], a, t, c] *: tail =>
+    case TypedColumn[n, a, t, c] *: tail =>
       TypedColumn[n, a, t, c] *: Reify[tail]
 
 // WARNING: it doesn't work if there's a type with given instance not
@@ -107,22 +105,22 @@ inline def reifyT[C <: Tuple]: Reify[C] =
   inline erasedValue[C] match
     case _: EmptyTuple =>
       EmptyTuple
-    case _: (TypedColumn[IsSingleton[n], a, t, c] *: tail) =>
+    case _: (TypedColumn[n, a, t, c] *: tail) =>
       TypedColumn[n, a, t, c](constValue[n], summonInline[IsColumn[a]]) *: reifyT[tail]
 
 type ReifyN[C <: NonEmptyTuple] <: NonEmptyTuple =
   C match
-    case TypedColumn[IsSingleton[n], a, t, c] *: EmptyTuple =>
+    case TypedColumn[n, a, t, c] *: EmptyTuple =>
       TypedColumn[n, a, t, c] *: EmptyTuple
-    case TypedColumn[IsSingleton[n], a, t, c] *: h2 *: tail =>
+    case TypedColumn[n, a, t, c] *: h2 *: tail =>
       TypedColumn[n, a, t, c] *: ReifyN[h2 *: tail]
 
 /** Reify a homogenous tuple of `TypedColumn` */
 inline def reifyNT[C <: NonEmptyTuple]: ReifyN[C] =
   inline erasedValue[C] match
-    case _: (TypedColumn[IsSingleton[n], a, t, c] *: EmptyTuple) =>
+    case _: (TypedColumn[n, a, t, c] *: EmptyTuple) =>
       TypedColumn[n, a, t, c](constValue[n], summonInline[IsColumn[a]]) *: EmptyTuple
-    case _: (TypedColumn[IsSingleton[n], a, t, c] *: h2 *: tail) =>
+    case _: (TypedColumn[n, a, t, c] *: h2 *: tail) =>
       TypedColumn[n, a, t, c](constValue[n], summonInline[IsColumn[a]]) *: reifyNT[h2 *: tail]
 
 type GetNames[C <: Tuple] <: Tuple =
@@ -141,8 +139,12 @@ inline def getNamesT[C <: Tuple]: GetNames[C] =
 
 type GetInNames[I <: NonEmptyTuple] <: NonEmptyTuple =
   I match
-    case TypedColumn.In[IsSingleton[n], ?, ?] *: EmptyTuple => n *: EmptyTuple
-    case TypedColumn.In[IsSingleton[n], ?, ?] *: t          => n *: GetInNames[t]
+    case TypedColumn.In[n, ?, ?] *: EmptyTuple =>
+      n match
+        case Singleton => n *: EmptyTuple
+    case TypedColumn.In[n, ?, ?] *: t =>
+      n match
+        case Singleton => n *: GetInNames[t]
 
 /** Match type to transform `(a, b, c, d)` of `TypedColumn.In` into `(((a, b), c), d)` (or `a ~ b ~
   * c ~ d`)
@@ -159,9 +161,6 @@ type TwiddleInGo[I <: Tuple, A <: Tuple] =
 
 // Generic operations
 
-/** Type-level unapply, checking if `T` is a singleton */
-type IsSingleton[T <: Singleton] = T
-
 /** If `A` in tuple `T` - return `True` branch, otherwise `False` branch */
 type IfIn[T <: Tuple, A, True, False] <: True | False = T match
   case EmptyTuple => False