new type traits: GenericHead and StripGenericParams

compiler/semmagic.nim

@@ -86,9 +86,32 @@ proc semInstantiationInfo(c: PContext, n: PNode): PNode =
   result.add(filename)
   result.add(line)
 
+proc toNode(t: PType, i: TLineInfo): PNode =
+  result = newNodeIT(nkType, i, t)
+
+const 
+  # these are types that use the bracket syntax for instantiation
+  # they can be subjected to the type traits `GenericHead` and 
+  # `Uninstantiated`
+  tyUserDefinedGenerics* = {tyGenericInst, tyGenericInvocation,
+                            tyUserTypeClassInst}
+
+  tyMagicGenerics* = {tySet, tySequence, tyArray, tyOpenArray}
+
+  tyGenericLike* = tyUserDefinedGenerics +
+                   tyMagicGenerics +
+                   {tyCompositeTypeClass}
+
+proc uninstantiate(t: PType): PType =
+  result = case t.kind
+    of tyMagicGenerics: t
+    of tyUserDefinedGenerics: t.base
+    of tyCompositeTypeClass: uninstantiate t.sons[1]
+    else: t
+
 proc evalTypeTrait(trait: PNode, operand: PType, context: PSym): PNode =
-  let typ = operand.skipTypes({tyTypeDesc})
-  case trait.sym.name.s.normalize
+  var typ = operand.skipTypes({tyTypeDesc})
+  case trait.sym.name.s
   of "name":
     result = newStrNode(nkStrLit, typ.typeToString(preferName))
     result.typ = newType(tyString, context)
@@ -97,6 +120,16 @@ proc evalTypeTrait(trait: PNode, operand: PType, context: PSym): PNode =
     result = newIntNode(nkIntLit, typ.len - ord(typ.kind==tyProc))
     result.typ = newType(tyInt, context)
     result.info = trait.info
+  of "GenericHead":
+    var res = uninstantiate(typ)
+    if res == typ and res.kind notin tyMagicGenerics:
+      localError(trait.info,
+        "GenericHead expects a generic type. The given type was " &
+        typeToString(typ))
+      return newType(tyError, context).toNode(trait.info)
+    result = res.base.toNode(trait.info)
+  of "StripGenericParams":
+    result = uninstantiate(typ).toNode(trait.info)
   else:
     internalAssert false
 

compiler/semtypes.nim

@@ -1226,6 +1226,13 @@ proc fixupTypeOf(c: PContext, prev: PType, typExpr: PNode) =
     result.sym = prev.sym
     assignType(prev, result)
 
+proc symFromExpectedTypeNode(c: PContext, n: PNode): PSym =
+  if n.kind == nkType:
+    result = symFromType(n.typ, n.info)
+  else:
+    localError(n.info, "xx")
+    result = errorSym(c, n)
+
 proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
   result = nil
   when defined(nimsuggest):
@@ -1316,7 +1323,9 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
     result = semTypeNode(c, whenResult, prev)
   of nkBracketExpr:
     checkMinSonsLen(n, 2)
-    var s = semTypeIdent(c, n.sons[0])
+    var head = n.sons[0]
+    var s = if head.kind notin nkCallKinds: semTypeIdent(c, head)
+            else: symFromExpectedTypeNode(c, semExpr(c, head))
     case s.magic
     of mArray: result = semArray(c, n, prev)
     of mOpenArray: result = semContainer(c, n, tyOpenArray, "openarray", prev)

lib/pure/typetraits.nim

@@ -19,7 +19,7 @@ proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ##
   ##   import typetraits
   ##
-  ##   proc `$`*[T](some:typedesc[T]): string = name(T)
+  ##   proc `$`*(T: typedesc): string = name(T)
   ##
   ##   template test(x): stmt =
   ##     echo "type: ", type(x), ", value: ", x
@@ -31,6 +31,21 @@ proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ##   test(@['A','B'])
   ##   # --> type: seq[char], value: @[A, B]
 
-
 proc arity*(t: typedesc): int {.magic: "TypeTrait".}
   ## Returns the arity of the given type
+
+proc GenericHead*(t: typedesc): typedesc {.magic: "TypeTrait".}
+  ## Accepts an instantiated generic type and returns its
+  ## uninstantiated form.
+  ##
+  ## For example:
+  ##   seq[int].GenericHead will be just seq
+  ##   seq[int].GenericHead[float] will be seq[float]
+  ##
+  ## A compile-time error will be produced if the supplied type
+  ## is not generic
+
+proc StripGenericParams*(t: typedesc): typedesc {.magic: "TypeTrait".}
+  ## This trait is similar to `GenericHead`, but instead of producing
+  ## error for non-generic types, it will just return them unmodified
+