typedesc and expr params

types are now valid proc/template/macro params and you can overload over them:
proc foo(T: typedesc)        # accept any type
proc foo(T: typedesc{int}) # overload specifically for int
proc foo(T: typedesc{int or float or Callable}) # overload for any type matching the constraints

expr{type} is a param expecting compile time value of the designated type (or type class).

when typedesc or expr params are used with a proc, the proc will be instantiated once
for each unique type/value used as parameter.

compiler/ast.nim

@@ -711,6 +711,15 @@ proc `[]`*(n: PNode, i: int): PNode {.inline.} =
 var emptyNode* = newNode(nkEmpty)
 # There is a single empty node that is shared! Do not overwrite it!
 
+proc linkTo*(t: PType, s: PSym): PType {.discardable.} =
+  t.sym = s
+  s.typ = t
+  result = t
+
+proc linkTo*(s: PSym, t: PType): PSym {.discardable.} =
+  t.sym = s
+  s.typ = t
+  result = s
 
 const                         # for all kind of hash tables:
   GrowthFactor* = 2           # must be power of 2, > 0

compiler/ccgcalls.nim

@@ -130,6 +130,7 @@ proc genPrefixCall(p: BProc, le, ri: PNode, d: var TLoc) =
   var length = sonsLen(ri)
   for i in countup(1, length - 1):
     assert(sonsLen(typ) == sonsLen(typ.n))
+    if ri.sons[i].typ.isCompileTimeOnly: continue
     if i < sonsLen(typ):
       assert(typ.n.sons[i].kind == nkSym)
       app(pl, genArg(p, ri.sons[i], typ.n.sons[i].sym))

compiler/ccgtypes.nim

@@ -7,6 +7,8 @@
 #    distribution, for details about the copyright.
 #
 
+# included from cgen.nim
+
 # ------------------------- Name Mangling --------------------------------
 
 proc mangle(name: string): string = 
@@ -48,6 +50,9 @@ proc mangleName(s: PSym): PRope =
     app(result, toRope(s.id))
     s.loc.r = result
 
+proc isCompileTimeOnly(t: PType): bool =
+  result = t.kind in {tyTypedesc, tyExpr}
+
 proc getTypeName(typ: PType): PRope = 
   if (typ.sym != nil) and ({sfImportc, sfExportc} * typ.sym.flags != {}) and
       (gCmd != cmdCompileToLLVM): 
@@ -187,6 +192,7 @@ proc genProcParams(m: BModule, t: PType, rettype, params: var PRope,
   for i in countup(1, sonsLen(t.n) - 1): 
     if t.n.sons[i].kind != nkSym: InternalError(t.n.info, "genProcParams")
     var param = t.n.sons[i].sym
+    if isCompileTimeOnly(param.typ): continue
     fillLoc(param.loc, locParam, param.typ, mangleName(param), OnStack)
     app(params, getParamTypeDesc(m, param.typ, check))
     if ccgIntroducedPtr(param): 
@@ -206,8 +212,8 @@ proc genProcParams(m: BModule, t: PType, rettype, params: var PRope,
       arr = arr.sons[0]
     if i < sonsLen(t.n) - 1: app(params, ", ")
   if (t.sons[0] != nil) and isInvalidReturnType(t.sons[0]): 
-    if params != nil: app(params, ", ")
     var arr = t.sons[0]
+    if params != nil: app(params, ", ")
     app(params, getTypeDescAux(m, arr, check))
     if (mapReturnType(t.sons[0]) != ctArray) or (gCmd == cmdCompileToLLVM): 
       app(params, "*")

compiler/cgen.nim

@@ -606,6 +606,7 @@ proc genProcAux(m: BModule, prc: PSym) =
         res.loc.s = OnUnknown
   for i in countup(1, sonsLen(prc.typ.n) - 1): 
     var param = prc.typ.n.sons[i].sym
+    if param.typ.isCompileTimeOnly: continue
     assignParam(p, param)
   closureSetup(p, prc)
   genStmts(p, prc.getBody) # modifies p.locals, p.init, etc.

compiler/msgs.nim

@@ -663,7 +663,7 @@ proc InternalError*(errMsg: string) =
   writeContext(UnknownLineInfo())
   rawMessage(errInternal, errMsg)
 
-template AssertNotNull*(e: expr): expr =
+template AssertNotNil*(e: expr): expr =
   if(e == nil): InternalError($InstantiationInfo())
   e
 

compiler/sem.nim

@@ -91,7 +91,7 @@ proc semConstExpr(c: PContext, n: PNode): PNode =
 
 proc semAndEvalConstExpr(c: PContext, n: PNode): PNode = 
   result = semConstExpr(c, n)
-  
+
 include seminst, semcall
 
 proc semAfterMacroCall(c: PContext, n: PNode, s: PSym): PNode = 

compiler/semdata.nim

@@ -180,14 +180,16 @@ proc addToLib(lib: PLib, sym: PSym) =
   sym.annex = lib
 
 proc makePtrType(c: PContext, baseType: PType): PType = 
-  if (baseType == nil): InternalError("makePtrType")
   result = newTypeS(tyPtr, c)
-  addSon(result, baseType)
+  addSon(result, baseType.AssertNotNil)
 
 proc makeVarType(c: PContext, baseType: PType): PType = 
-  if (baseType == nil): InternalError("makeVarType")
   result = newTypeS(tyVar, c)
-  addSon(result, baseType)
+  addSon(result, baseType.AssertNotNil)
+
+proc makeTypeDesc*(c: PContext, typ: PType): PType =
+  result = newTypeS(tyTypeDesc, c)
+  result.addSon(typ.AssertNotNil)
 
 proc newTypeS(kind: TTypeKind, c: PContext): PType = 
   result = newType(kind, getCurrOwner())

compiler/semexprs.nim

@@ -10,6 +10,8 @@
 # this module does the semantic checking for expressions
 # included from sem.nim
 
+proc semExprOrTypedesc(c: PContext, n: PNode): PNode
+
 proc semTemplateExpr(c: PContext, n: PNode, s: PSym, semCheck = true): PNode = 
   markUsed(n, s)
   pushInfoContext(n.info)
@@ -93,6 +95,8 @@ proc semSym(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
     # if a proc accesses a global variable, it is not side effect free:
     if sfGlobal in s.flags:
       incl(c.p.owner.flags, sfSideEffect)
+    elif s.kind == skParam and s.typ.kind == tyExpr:
+      return s.typ.n
     elif s.owner != c.p.owner and s.owner.kind != skModule and 
         c.p.owner.typ != nil and not IsGenericRoutine(s.owner):
       c.p.owner.typ.callConv = ccClosure
@@ -111,7 +115,7 @@ proc semSym(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
   else:
     markUsed(n, s)
     result = newSymNode(s, n.info)
-
+  
 proc checkConversionBetweenObjects(info: TLineInfo, castDest, src: PType) =
   var diff = inheritanceDiff(castDest, src)
   if diff == high(int):
@@ -247,16 +251,31 @@ proc semIs(c: PContext, n: PNode): PNode =
   else:
     GlobalError(n.info, errXExpectsTwoArguments, "is")
 
+proc semExprOrTypedesc(c: PContext, n: PNode): PNode =
+  # XXX: Currently, semExprWithType will return the same type
+  # for nodes such as (100) or (int). 
+  # This is inappropriate. The type of the first expression
+  # should be "int", while the type of the second one should 
+  # be typeDesc(int).
+  # Ideally, this should be fixed in semExpr, but right now 
+  # there are probably users that depend on the present behavior.
+  # XXX: Investigate current uses of efAllowType and fix them to
+  # work with tyTypeDesc.
+  result = semExprWithType(c, n, {efAllowType})
+  if result.kind == nkSym and result.sym.kind == skType and
+     result.typ.kind != tyTypeDesc:
+    result.typ = makeTypeDesc(c, result.typ)
+
 proc semOpAux(c: PContext, n: PNode) =
   for i in countup(1, sonsLen(n) - 1):
     var a = n.sons[i]
     if a.kind == nkExprEqExpr and sonsLen(a) == 2: 
       var info = a.sons[0].info
       a.sons[0] = newIdentNode(considerAcc(a.sons[0]), info)
-      a.sons[1] = semExprWithType(c, a.sons[1], {efAllowType})
+      a.sons[1] = semExprOrTypedesc(c, a.sons[1])
       a.typ = a.sons[1].typ
     else:
-      n.sons[i] = semExprWithType(c, a, {efAllowType})
+      n.sons[i] = semExprOrTypedesc(c, a)
     
 proc overloadedCallOpr(c: PContext, n: PNode): PNode = 
   # quick check if there is *any* () operator overloaded:
@@ -822,7 +841,7 @@ proc semDeref(c: PContext, n: PNode): PNode =
   of tyRef, tyPtr: n.typ = t.sons[0]
   else: result = nil
   #GlobalError(n.sons[0].info, errCircumNeedsPointer) 
-  
+
 proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
   ## returns nil if not a built-in subscript operator; also called for the
   ## checking of assignments
@@ -833,7 +852,7 @@ proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
     result.add(x[0])
     return
   checkMinSonsLen(n, 2)
-  n.sons[0] = semExprWithType(c, n.sons[0], flags - {efAllowType})
+  n.sons[0] = semExprOrTypedesc(c, n.sons[0])
   var arr = skipTypes(n.sons[0].typ, {tyGenericInst, tyVar, tyPtr, tyRef})
   case arr.kind
   of tyArray, tyOpenArray, tyArrayConstr, tySequence, tyString, tyCString: 
@@ -848,6 +867,11 @@ proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
       result = n
       result.typ = elemType(arr)
     #GlobalError(n.info, errIndexTypesDoNotMatch)
+  of tyTypeDesc:
+    result = n.sons[0] # The result so far is a tyTypeDesc bound to
+                       # a tyGenericBody. The line below will substitute
+                       # it with the instantiated type.
+    result.typ.sons[0] = semTypeNode(c, n, nil).linkTo(result.sym)
   of tyTuple: 
     checkSonsLen(n, 2)
     n.sons[0] = makeDeref(n.sons[0])
@@ -1024,7 +1048,7 @@ proc semExpandToAst(c: PContext, n: PNode, magicSym: PSym,
     markUsed(n, expandedSym)
 
     for i in countup(1, macroCall.len-1):
-      macroCall.sons[i] = semExprWithType(c, macroCall[i], {efAllowType})
+      macroCall.sons[i] = semExprWithType(c, macroCall[i], {})
 
     # Preserve the magic symbol in order to be handled in evals.nim
     n.sons[0] = newSymNode(magicSym, n.info)
@@ -1287,6 +1311,12 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
   of nkBind:
     Message(n.info, warnDeprecated, "bind")
     result = semExpr(c, n.sons[0], flags)
+  of nkTypeOfExpr:
+    var typ = semTypeNode(c, n, nil)
+    if typ.sym == nil:
+      typ = copyType(typ, typ.owner, true)
+      typ.linkTo(newSym(skType, getIdent"typedesc", typ.owner))
+    result = newSymNode(typ.sym, n.info)
   of nkCall, nkInfix, nkPrefix, nkPostfix, nkCommand, nkCallStrLit: 
     # check if it is an expression macro:
     checkMinSonsLen(n, 1)

compiler/seminst.nim

@@ -20,7 +20,7 @@ proc instantiateGenericParamList(c: PContext, n: PNode, pt: TIdTable,
     if a.kind != nkSym: 
       InternalError(a.info, "instantiateGenericParamList; no symbol")
     var q = a.sym
-    if q.typ.kind notin {tyTypeDesc, tyGenericParam, tyTypeClass}: continue 
+    if q.typ.kind notin {tyTypeDesc, tyGenericParam, tyTypeClass, tyExpr}: continue
     var s = newSym(skType, q.name, getCurrOwner())
     s.info = q.info
     s.flags = s.flags + {sfUsed, sfFromGeneric}

compiler/semtypes.nim

@@ -180,6 +180,8 @@ proc semTypeIdent(c: PContext, n: PNode): PSym =
     result = qualifiedLookup(c, n, {checkAmbiguity, checkUndeclared})
     if result != nil:
       markUsed(n, result)
+      if result.kind == skParam and result.typ.kind == tyTypeDesc:
+        return result.typ.sons[0].sym
       if result.kind != skType: GlobalError(n.info, errTypeExpected)
       if result.typ.kind != tyGenericParam:
         # XXX get rid of this hack!
@@ -490,9 +492,20 @@ proc paramTypeClass(c: PContext, paramType: PType, procKind: TSymKind):
   # if id is not nil, the generic param will bind just once (see below)
   case paramType.kind:
   of tyExpr:
-    # proc(a, b: expr)
     if procKind notin {skTemplate, skMacro}:
-      result.typ = newTypeS(tyGenericParam, c)
+      if paramType.sonsLen == 0:
+        # proc(a, b: expr)
+        # no constraints, treat like generic param
+        result.typ = newTypeS(tyGenericParam, c)
+      else:
+        # proc(a: expr{string}, b: expr{nkLambda})
+        # overload on compile time values and AST trees
+        result.typ = newTypeS(tyExpr, c)
+        result.typ.sons = paramType.sons
+  of tyTypeDesc:
+    if procKind notin {skTemplate, skMacro}:
+      result.typ = newTypeS(tyTypeDesc, c)
+      result.typ.sons = paramType.sons
   of tyDistinct:
     # type T1 = distinct expr
     # type S1 = distinct Sortable
@@ -565,7 +578,7 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
         if genericParams == nil:
           # genericParams is nil when the proc is being instantiated
           # the resolved type will be in scope then
-          endingType = SymtabGet(c.tab, paramTypId).AssertNotNull.typ
+          endingType = SymtabGet(c.tab, paramTypId).AssertNotNil.typ
         else:
           block addImplicitGeneric:
             # is this a bindOnce type class already present in the param list?
@@ -664,9 +677,10 @@ proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
     if s.ast == nil: GlobalError(n.info, errCannotInstantiateX, s.name.s)
     result = instGenericContainer(c, n, result)
 
-proc semExpandToType(c: PContext, n: PNode, sym: PSym): PType =
+proc semTypeFromMacro(c: PContext, n: PNode): PType =
   # Expands a macro or template until a type is returned
   # results in GlobalError if the macro expands to something different
+  var sym = expectMacroOrTemplateCall(c, n)
   markUsed(n, sym)
   case sym.kind
   of skMacro:
@@ -676,7 +690,7 @@ proc semExpandToType(c: PContext, n: PNode, sym: PSym): PType =
   else:
     GlobalError(n.info, errXisNoMacroOrTemplate, n.renderTree)
 
-proc semTypeNode(c: PContext, n: PNode, prev: PType): PType = 
+proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
   result = nil
   if gCmd == cmdIdeTools: suggestExpr(c, n)
   case n.kind
@@ -703,9 +717,13 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
         result.addSon(t2)
         result.flags.incl(if op == ord(wAnd): tfAll else: tfAny)
     else:
-      # expand macros and templates
-      var expandedSym = expectMacroOrTemplateCall(c, n)
-      result = semExpandToType(c, n, expandedSym)
+      result = semTypeFromMacro(c, n)
+  of nkCurlyExpr:
+    result = semTypeNode(c, n.sons[0], nil)
+    if result != nil:
+      result = copyType(result, getCurrOwner(), false)
+      for i in countup(1, n.len - 1):
+        result.addSon(semTypeNode(c, n.sons[i], nil))
   of nkWhenStmt:
     var whenResult = semWhen(c, n, false)
     if whenResult.kind == nkStmtList: whenResult.kind = nkStmtListType

compiler/sigmatch.nim

@@ -35,7 +35,7 @@ type
   
   TTypeRelation* = enum      # order is important!
     isNone, isConvertible, isIntConv, isSubtype, 
-    isGeneric, 
+    isGeneric
     isEqual
   
 proc initCandidateAux(c: var TCandidate, callee: PType) {.inline.} = 
@@ -208,16 +208,32 @@ proc tupleRel(mapping: var TIdTable, f, a: PType): TTypeRelation =
         var y = a.n.sons[i].sym
         if x.name.id != y.name.id: return isNone
 
-proc matchTypeClass(mapping: var TIdTable, f, a: PType): TTypeRelation = 
-  result = isNone
-  let son = f.sons[0]
-  if son.kind == a.kind:
-    result = isGeneric
-  elif son.kind == tyGenericBody:
-    if a.kind == tyGenericInst and a.sons[0] == son:
-      result = isGeneric
-      put(mapping, f, a)
+proc matchTypeClass(mapping: var TIdTable, f, a: PType): TTypeRelation =
+  for i in countup(0, f.sonsLen - 1):
+    let son = f.sons[i]
+    var match = son.kind == a.kind
 
+    if not match:
+      case son.kind
+      of tyGenericBody:
+        if a.kind == tyGenericInst and a.sons[0] == son:
+          match = true
+          put(mapping, f, a)
+      of tyTypeClass:
+        match = matchTypeClass(mapping, son, a) == isGeneric
+      else: nil
+
+    if tfAny in f.flags:
+      if match == true:
+        return isGeneric
+    else:
+      if match == false:
+        return isNone
+
+  # if the loop finished without returning, either all constraints matched
+  # or none of them matched.
+  result = if tfAny in f.flags: isNone else: isGeneric
+  
 proc procTypeRel(mapping: var TIdTable, f, a: PType): TTypeRelation =
   proc inconsistentVarTypes(f, a: PType): bool {.inline.} =
     result = f.kind != a.kind and (f.kind == tyVar or a.kind == tyVar)
@@ -347,8 +363,6 @@ proc typeRel(mapping: var TIdTable, f, a: PType): TTypeRelation =
         result = typeRel(mapping, f.sons[0], a.sons[0])
         if result < isGeneric: result = isNone
     else: nil
-  of tyTypeClass:
-    result = matchTypeClass(mapping, f, a)
   of tyOrdinal:
     if isOrdinalType(a):
       var x = if a.kind == tyOrdinal: a.sons[0] else: a
@@ -469,7 +483,19 @@ proc typeRel(mapping: var TIdTable, f, a: PType): TTypeRelation =
       result = isGeneric
     else: 
       result = typeRel(mapping, x, a) # check if it fits
-  of tyExpr, tyStmt, tyTypeDesc:
+  of tyTypeClass:
+    result = matchTypeClass(mapping, f, a)
+    if result == isGeneric: put(mapping, f, a)
+  of tyTypeDesc:
+    if a.kind == tyTypeDesc:
+      if f.sonsLen == 0:
+        result = isGeneric
+      else:
+        result = matchTypeClass(mapping, f, a.sons[0])
+      if result == isGeneric: put(mapping, f, a)
+    else:
+      result = isNone
+  of tyExpr, tyStmt:
     result = isGeneric
   else: internalError("typeRel(" & $f.kind & ')')
   
@@ -512,9 +538,34 @@ proc userConvMatch(c: PContext, m: var TCandidate, f, a: PType,
       inc(m.convMatches)
       return 
 
+
 proc ParamTypesMatchAux(c: PContext, m: var TCandidate, f, a: PType, 
                         arg, argOrig: PNode): PNode =
-  var r = typeRel(m.bindings, f, a)
+  var r: TTypeRelation
+  if f.kind == tyExpr:
+    if f.sonsLen == 0:
+      r = isGeneric
+    else:
+      let match = matchTypeClass(m.bindings, f, a)
+      if match != isGeneric: r = isNone
+      else:
+        # XXX: Ideally, this should happen much earlier somewhere near 
+        # semOpAux, but to do that, we need to be able to query the 
+        # overload set to determine whether compile-time value is expected
+        # for the param before entering the full-blown sigmatch algorithm.
+        # This is related to the immediate pragma since querying the
+        # overload set could help there too.
+        var evaluated = c.semConstExpr(c, arg)
+        if evaluated != nil:
+          r = isGeneric
+          arg.typ = newTypeS(tyExpr, c)
+          arg.typ.n = evaluated
+        
+    if r == isGeneric:
+      put(m.bindings, f, arg.typ)
+  else:
+    r = typeRel(m.bindings, f, a)
+  
   case r
   of isConvertible: 
     inc(m.convMatches)

compiler/types.nim

@@ -48,15 +48,15 @@ proc equalParams*(a, b: PNode): TParamsEquality
 proc isOrdinalType*(t: PType): bool
 proc enumHasHoles*(t: PType): bool
 const 
-  abstractPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyDistinct,
+  abstractPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyDistinct, tyOrdinal,
                    tyConst, tyMutable}
-  abstractVar* = {tyVar, tyGenericInst, tyDistinct,
+  abstractVar* = {tyVar, tyGenericInst, tyDistinct, tyOrdinal,
                   tyConst, tyMutable}
-  abstractRange* = {tyGenericInst, tyRange, tyDistinct,
+  abstractRange* = {tyGenericInst, tyRange, tyDistinct, tyOrdinal,
                     tyConst, tyMutable}
-  abstractVarRange* = {tyGenericInst, tyRange, tyVar, tyDistinct,
+  abstractVarRange* = {tyGenericInst, tyRange, tyVar, tyDistinct, tyOrdinal,
                        tyConst, tyMutable}
-  abstractInst* = {tyGenericInst, tyDistinct, tyConst, tyMutable}
+  abstractInst* = {tyGenericInst, tyDistinct, tyConst, tyMutable, tyOrdinal}
 
   skipPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyConst, tyMutable}
 
@@ -142,7 +142,7 @@ proc skipTypes(t: PType, kinds: TTypeKinds): PType =
 proc isOrdinalType(t: PType): bool = 
   assert(t != nil)
   result = (t.Kind in {tyChar, tyInt..tyInt64, tyBool, tyEnum}) or
-      (t.Kind in {tyRange, tyConst, tyMutable, tyGenericInst}) and
+      (t.Kind in {tyRange, tyOrdinal, tyConst, tyMutable, tyGenericInst}) and
        isOrdinalType(t.sons[0])
 
 proc enumHasHoles(t: PType): bool = 
@@ -730,8 +730,10 @@ proc SameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
       if a.kind != b.kind: return false
   case a.Kind
   of tyEmpty, tyChar, tyBool, tyNil, tyPointer, tyString, tyCString,
-     tyInt..tyBigNum, tyExpr, tyStmt, tyTypeDesc, tyOrdinal:
+     tyInt..tyBigNum, tyStmt:
     result = true
+  of tyExpr:
+    result = ExprStructuralEquivalent(a.n, b.n)
   of tyObject:
     IfFastObjectTypeCheckFailed(a, b):
       CycleCheck()
@@ -740,7 +742,7 @@ proc SameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     CycleCheck()
     if c.cmp == dcEq: result = sameDistinctTypes(a, b)
     else: result = sameTypeAux(a.sons[0], b.sons[0], c)
-  of tyEnum, tyForward, tyProxy, tyTypeClass:
+  of tyEnum, tyForward, tyProxy:
     # XXX generic enums do not make much sense, but require structural checking
     result = a.id == b.id
   of tyTuple:
@@ -749,7 +751,8 @@ proc SameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
   of tyGenericInst: result = sameTypeAux(lastSon(a), lastSon(b), c)
   of tyGenericParam, tyGenericInvokation, tyGenericBody, tySequence,
      tyOpenArray, tySet, tyRef, tyPtr, tyVar, tyArrayConstr,
-     tyArray, tyProc, tyConst, tyMutable, tyVarargs, tyIter: 
+     tyArray, tyProc, tyConst, tyMutable, tyVarargs, tyIter,
+     tyOrdinal, tyTypeDesc, tyTypeClass:
     if sonsLen(a) == sonsLen(b):
       CycleCheck()
       result = true

tests/run/tmemoization.nim

@@ -0,0 +1,17 @@
+discard """
+  msg:    "test 1\ntest 2"
+  output: "TEST 1\nTEST 2\nTEST 2"
+"""
+
+import strutils
+
+proc foo(s: expr{string}): string =
+  static: echo s
+
+  const R = s.toUpper
+  return R
+  
+echo foo("test 1")
+echo foo("test 2")
+echo foo("test " & $2)
+

tests/run/ttypedesc1.nim

@@ -0,0 +1,35 @@
+import unittest
+
+type 
+  TFoo[T, U] = object
+    x: T
+    y: U
+
+proc foo(T: typedesc{float}, a: expr): string =
+  result = "float " & $(a.len > 5)
+
+proc foo(T: typedesc{TFoo}, a: int): string =
+  result = "TFoo "  & $(a)
+
+proc foo(T: typedesc{int or bool}): string =
+  var a: T
+  a = 10
+  result = "int or bool " & ($a)
+
+template foo(T: typedesc{seq}): expr = "seq"
+
+test "types can be used as proc params":
+  check foo(TFoo[int, float], 1000) == "TFoo 1000"
+  
+  var f = 10.0
+  check foo(float, "long string") == "float true"
+  check foo(type(f), [1, 2, 3]) == "float false"
+  
+  check foo(int) == "int or bool 10"
+
+  check foo(seq[int]) == "seq"
+  check foo(seq[TFoo[bool, string]]) == "seq"
+
+when false:
+  proc foo(T: typedesc{seq}, s: T) = nil
+