Implement new Nil model

TODO

@@ -63,8 +63,6 @@ HARD
 
   *Finish roles.
 
-   Study Nil and find the correct way to integrate it into Niecza.
-
    Add a JVM backend.
 
    Add a Parrot backend.

lib/CORE.setting

@@ -434,11 +434,11 @@ my class Submethod is Routine { }
 my class WhateverCode is Block { }
 
 my class ClassHOW {
-    method isa(Mu $obj, Mu $type) { Q:CgOp {
+    method isa(\$obj, \$type) { Q:CgOp {
         (box Bool (obj_isa (@ {$obj}) (obj_llhow (@ {$type}))))
     } }
-    method does(Mu $obj, Mu $role) { self.isa($obj, $role) } #no roles yet
-    method can(Mu $obj, $name) { Q:CgOp {
+    method does(\$obj, \$role) { self.isa($obj, $role) } #no roles yet
+    method can(\$obj, $name) { Q:CgOp {
         (box Bool (obj_can (@ {$obj}) (obj_getstr {$name})))
     } }
 }
@@ -802,6 +802,13 @@ my class EMPTY { }
 my class List { ... }
 my class Array { ... }
 
+my class Nil is Cool {
+    method new() { Nil }
+    method iterator() { ().iterator }
+    method gist() { 'Nil' }
+    method Str() { '' }
+}
+
 my class Parcel is Cool {
     method flat() { self.iterator.flat }
     method list() { self.iterator.list }
@@ -817,8 +824,10 @@ my class Parcel is Cool {
 
     method unwrap-single(@self:) { Q:CgOp {
         (letn p (unbox fvarlist (@ {@self}))
-          (ternary (== (i 1) (fvarlist_length (l p)))
-            (fvarlist_item (i 0) (l p)) {@self}))
+              l (fvarlist_length (l p))
+          (ternary (== (i 0) (l l)) {Nil}
+            (ternary (== (i 1) (l l))
+              (fvarlist_item (i 0) (l p)) {@self})))
     } }
 
     method Capture () {
@@ -856,8 +865,6 @@ my class Parcel is Cool {
     }
 }
 
-constant Nil = Q:CgOp { (newrwlistvar (@ (box Parcel (fvarlist_new)))) };
-
 my class List is Cool {
     has @!items;
     has @!rest;

lib/Kernel.cs

@@ -127,6 +127,9 @@ public sealed class SimpleVariable: Variable {
             if (!rw) {
                 throw new NieczaException("Writing to readonly scalar");
             }
+            if (v == Kernel.NilP) {
+                v = type.initObject;
+            }
             if (!v.Does(type)) {
                 throw new NieczaException("Nominal type check failed for scalar store; got " + v.mo.name + ", needed " + type.name + " or subtype");
             }
@@ -643,6 +646,11 @@ public sealed class RuntimeUnit {
 
             mo.fixups_from = from;
 
+            if (this.name == "CORE" && name == "Nil") {
+                // this anomalous type object is iterable
+                mo.typeVar = Kernel.NewRWListVar(mo.typeObject);
+            }
+
             return mo;
         }
 
@@ -1006,6 +1014,7 @@ public class SubInfo {
                     src = pos[posc++];
                     goto gotit;
                 }
+get_default:
                 if ((flags & SIG_F_HASDEFAULT) != 0) {
                     Frame thn = Kernel.GetInferiorRoot()
                         .MakeChild(th, (SubInfo) rbuf[rbase + 1 + names],
@@ -1047,6 +1056,13 @@ public class SubInfo {
                     bool rw     = ((flags & SIG_F_READWRITE) != 0) && !islist;
                     P6any srco  = src.Fetch();
 
+                    // XXX: in order for calling methods on Nil to work,
+                    // self needs to be ignored here.
+                    if (srco == Kernel.NilP && obj_src != -1 &&
+                            (flags & SIG_F_INVOCANT) == 0) {
+                        obj_src = -1;
+                        goto get_default;
+                    }
                     if (!srco.Does(type)) {
                         if (quiet) return null;
                         if (srco.mo.HasMRO(Kernel.JunctionMO) && obj_src != -1) {
@@ -2981,6 +2997,7 @@ public class Kernel {
         public static STable CaptureMO;
         public static STable GatherIteratorMO;
         public static STable IterCursorMO;
+        public static P6any NilP;
         public static P6any AnyP;
         public static P6any ArrayP;
         public static P6any EMPTYP;

src/NieczaPassSimplifier.pm6

@@ -85,6 +85,7 @@ sub do_builtin($name, $expect) { sub ($body, $nv, $invname, $op) { #OK not used
 sub do_return_take($body, $nv, $invname, $op) { #OK not used
     return $op unless defined my $args = no_named_params($op);
     my $parcel = ($args == 1 ?? $args[0] !!
+        $args == 0 ?? ::Op::Lexical.new(name => 'Nil') !!
         ::Op::CallSub.new(invocant => ::Op::Lexical.new(name => '&infix:<,>'),
             positionals => [@$args]));
     return ($invname eq '&take' ??

src/niecza

@@ -157,6 +157,21 @@ class Op::IndirectVar is Op {
 
 
 augment class NieczaActions {
+method circumfix:sym<( )> ($/) {
+    my @kids = @( $<semilist>.ast );
+    if @kids == 1 && @kids[0].^isa(::Op::WhateverCode) {
+        # XXX in cases like * > (2 + *), we *don't* want the parens to disable
+        # syntactic specialization, since they're required for grouping
+        make @kids[0];
+    } elsif !@kids {
+        # an empty StatementList returns Nil, but () needs to be defined...
+        make ::Op::Paren.new(|node($/), inside =>
+            ::Op::SimpleParcel.new(items => []));
+    } else {
+        make ::Op::StatementList.new(|node($/), children => @kids);
+    }
+}
+
 method process_name($/, :$declaring, :$defer, :$clean) {
     return () unless defined $/;
 

t/spectest.data

@@ -10,6 +10,7 @@ S02-builtin_data_types/bool.t
 S02-builtin_data_types/catch_type_cast_mismatch.t
 S02-builtin_data_types/flattening.t
 S02-builtin_data_types/nested_pairs.t
+S02-builtin_data_types/nil.t
 S02-builtin_data_types/parsing-bool.t
 S02-builtin_data_types/subscripts_and_context.t
 S02-lexical-conventions/unicode.t