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Add impl(type, name) to retrieve a particular implementation of a trait decl

This avoids having to create a temporary variable when you know what
implementation your want for a generic constant or function with generic return
value.

Re-use the impl keyword for this expression to avoid invalidating existing
programs.

--- a/doc/lang.txt

+++ b/doc/lang.txt

@@ -1239,7 +1239,8 @@

         5.2.3. Atomic Expressions:

                 atomicexpr:     ident | gap | literal | "(" expr ")" | 

-                                "sizeof" "(" type ")" | castexpr

+                                "sizeof" "(" type ")" | castexpr |

+                                "impl" "(" name "," type ")"

                 castexpr:       "(" expr ":" type ")"

                 gap:            "_"

@@ -1265,6 +1266,10 @@

             match, again, given that it is never read from in the body of the

             match.

+            An impl expression chooses the implementation of the given trait

+            declaration for the given type. It is useful for refering to trait

+            declarations in a generic context.

+

         5.2.4. Cast Expressions:

             Cast expressions convert a value from one type to another.  Some

@@ -2012,7 +2017,8 @@

                 postepxr "#" |

                 atomicexpr

-    atomicexpr: ident | literal | "(" expr ")" | "sizeof" "(" type ")"

+    atomicexpr: ident | literal | "(" expr ")" | "sizeof" "(" type ")" |

+                "(" expr ":" type ")" | "impl" "(" name "," type ")"

     /* literals */

     literal:    funclit | seqlit | tuplit | simplelit

--- a/parse/gram.y

+++ b/parse/gram.y

@@ -380,7 +380,7 @@

 name    : Tident {$$ = mkname($1->loc, $1->id);}

 	| Tident Tdot Tident {

-		$$ = mkname($3->loc, $3->id); setns($$, $1->id);

+		$$ = mknsname($3->loc, $1->id, $3->id);

 	}

 	;

@@ -755,6 +755,10 @@

 	}

 	| Tsizeof Toparen type Tcparen

 	{$$ = mkexpr($1->loc, Osize, mkpseudodecl($1->loc, $3), NULL);}

+	| Timpl Toparen name Tcomma type Tcparen {

+		$$ = mkexpr($1->loc, Ovar, $3, NULL);

+		$$->expr.param = $5;

+	}

 	;

 tupbody : tuphead tuprest

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -1199,13 +1199,16 @@

 	if (s->decl.ishidden)

 		fatal(n, "attempting to refer to hidden decl %s", ctxstr(st, n));

-	param = NULL;

+	param = n->expr.param;

 	if (s->decl.isgeneric) {

 		subst = mksubst();

+		if (param)

+			substput(subst, s->decl.trait->param, param);

 		t = tysubstmap(st, subst, tf(st, s->decl.type), s->decl.type);

-		if (s->decl.trait) {

+		if (s->decl.trait && !param) {

 			param = substget(subst, s->decl.trait->param);

-			delayedcheck(st, n, curstab());

+			if (!param)

+				fatal(n, "ambiguous trait decl %s", ctxstr(st, s));

 		}

 		substfree(subst);

 	} else {

@@ -1213,7 +1216,10 @@

 	}

 	n->expr.did = s->decl.did;

 	n->expr.isconst = s->decl.isconst;

-	n->expr.param = param;

+	if (param) {

+		n->expr.param = param;

+		delayedcheck(st, n, curstab());

+	}

 	if (s->decl.isgeneric && !st->ingeneric) {

 		t = tyfreshen(st, NULL, t);

 		addspecialization(st, n, curstab());

@@ -1484,6 +1490,7 @@

 	Node *s, *n;

 	Type *t, *b;

 	int isconst;

+	Stab *ns;

 	n = *np;

 	assert(n->type == Nexpr);

@@ -1650,9 +1657,14 @@

 		 * already done with this node, we can just return. */

 		if (n->expr.type)

 			return;

-		s = getdcl(curstab(), args[0]);

+		ns = curstab();

+		if (args[0]->name.ns)

+			ns = getns(file, args[0]->name.ns);

+		s = getdcl(ns, args[0]);

 		if (!s)

 			fatal(n, "undeclared var %s", ctxstr(st, args[0]));

+		if (n->expr.param && !s->decl.trait)

+			fatal(n, "var %s must refer to a trait decl", ctxstr(st, args[0]));

 		initvar(st, n, s);

 		break;

 	case Ogap: /* _ -> @a */

@@ -1812,7 +1824,7 @@

 		putdcl(file->file.globls, dcl);

 		htput(proto->decl.impls, n->impl.type, dcl);

 		dcl->decl.isconst = 1;

-		if (n->impl.type->type == Tygeneric || hasparams(n->impl.type)) {

+		if (ty->type == Tygeneric || hasparams(ty)) {

 			dcl->decl.isgeneric = 1;

 			lappend(&proto->decl.gimpl, &proto->decl.ngimpl, dcl);

 			lappend(&proto->decl.gtype, &proto->decl.ngtype, ty);

--- /dev/null

+++ b/test/implexpr.myr

@@ -1,0 +1,18 @@

+use std

+

+trait favorite @a =

+	Fav : @a

+;;

+impl favorite int =

+	Fav = 12

+;;

+impl favorite char =

+	Fav = 'z'

+;;

+impl favorite byte[:] =

+	Fav = "hello"

+;;

+

+const main = {

+	std.put("{},{},{}\n", impl(Fav, int), impl(Fav, char), impl(Fav, byte[:]))

+}

--- a/test/tests

+++ b/test/tests

@@ -164,4 +164,4 @@

 B nestedgoto	E	0

 B initializer	E	0

 B fmtalign	E	0

-

+B implexpr	P	12,z,hello