add extractGeneric: extractGeneric(Foo2[float, string], 0) is float

doc/manual.rst

@@ -4351,14 +4351,15 @@ type classes are called `bind many`:idx: types.
 
 Procs written with the implicitly generic style will often need to refer to the
 type parameters of the matched generic type. They can be easily accessed using
-the dot syntax:
+``sugar.extractGeneric`` or the dot syntax:
 
 .. code-block:: nim
   type Matrix[T, Rows, Columns] = object
     ...
 
   proc `[]`(m: Matrix, row, col: int): Matrix.T =
     m.data[col * high(Matrix.Columns) + row]
+    # we could've also used ``extractGeneric(Matrix, 0)``
 
 Alternatively, the `type` operator can be used over the proc params for similar
 effect when anonymous or distinct type classes are used.

lib/pure/sugar.nim

@@ -198,3 +198,34 @@ macro dump*(x: typed): untyped =
   let r = quote do:
     debugEcho `s`, " = ", `x`
   return r
+
+macro extractGeneric*(T: typedesc, index:static[int]): untyped =
+  ## extract generic type numbered ``index`` used to construct ``T``. Note:
+  ## ``-1`` returns ``Foo`` in ``Foo[T]``
+  runnableExamples:
+    type Foo[T1, T2]=object
+    doAssert extractGeneric(Foo[float, string], 0) is float
+    doAssert extractGeneric(Foo[float, string], 1) is string
+    doAssert extractGeneric(Foo[float, string], -1) is Foo
+
+  var impl = getTypeImpl(T)
+  expectKind(impl, nnkBracketExpr)
+  impl = impl[1]
+  while true:
+    case impl.kind
+      of nnkSym:
+        impl = impl.getImpl
+        continue
+      of nnkTypeDef:
+        impl = impl[2]
+        continue
+      of nnkBracketExpr:
+        if index == -1:
+          impl=impl[0] #return `Foo` in `Foo[T]`
+        else:
+          impl=impl[1+index]  #return `T` in `Foo[T]` (when index = 0)
+        break
+      else:
+        error "internal error: impl.kind: " & $impl.kind
+  impl
+

tests/stdlib/tsugar.nim

@@ -0,0 +1,21 @@
+discard """
+  file: "tsugar.nim"
+  output: ""
+"""
+import sugar
+import macros
+
+block extractGeneric:
+  type Foo[T1, T2]=object
+  type Foo2=Foo[float, string]
+  doAssert extractGeneric(Foo[float, string], 1) is string
+  doAssert extractGeneric(Foo2, 1) is string
+  # workaround for seq[int].T not working,
+  # see https://github.com/nim-lang/Nim/issues/8433
+  doAssert extractGeneric(seq[int], 0) is int
+  doAssert extractGeneric(seq[seq[string]], 0) is seq[string]
+  doAssert: not compiles(extractGeneric(seq[int], 1))
+  doAssert extractGeneric(seq[int], -1) is seq
+
+  type Foo3[T] = T
+  doAssert extractGeneric(Foo3[int], 0) is int