Bring back "pred" syntax for writing predicates for check

This commit reinstates the requirement that the predicate in a
"check" must be a manifest call to a special kind of function
declared with the new "pred" keyword instead of "fn". Preds must
have a boolean return type and can only call other preds; they
can't have any effects (as enforced by the typechecker).
The arguments to a predicate in a check expression must be
slot variables or literals.

7 files changed, 155 insertions, 22 deletions

diff --git a/src/comp/front/ast.rs b/src/comp/front/ast.rs
index cc7569e1..574ebd4a 100644
--- a/src/comp/front/ast.rs
+++ b/src/comp/front/ast.rs
@@ -346,7 +346,13 @@ type constr = spanned[constr_];
 
 type arg = rec(mode mode, @ty ty, ident ident, def_id id);
 type fn_decl = rec(vec[arg] inputs,
-                   @ty output);
+                   @ty output,
+                   purity purity);
+tag purity {
+    pure_fn;   // declared with "pred"
+    impure_fn; // declared with "fn"
+}
+
 type _fn = rec(fn_decl decl,
                proto proto,
                block body);
diff --git a/src/comp/front/lexer.rs b/src/comp/front/lexer.rs
index 9ef6ea27..4153533a 100644
--- a/src/comp/front/lexer.rs
+++ b/src/comp/front/lexer.rs
@@ -141,6 +141,7 @@ fn keyword_table() -> std.map.hashmap[str, token.token] {
     keywords.insert("auto", token.AUTO);
 
     keywords.insert("fn", token.FN);
+    keywords.insert("pred", token.PRED);
     keywords.insert("iter", token.ITER);
 
     keywords.insert("import", token.IMPORT);
diff --git a/src/comp/front/parser.rs b/src/comp/front/parser.rs
index 3ef16c71..e355290d 100644
--- a/src/comp/front/parser.rs
+++ b/src/comp/front/parser.rs
@@ -242,6 +242,7 @@ fn parse_proto(parser p) -> ast.proto {
     alt (p.peek()) {
         case (token.ITER) { p.bump(); ret ast.proto_iter; }
         case (token.FN) { p.bump(); ret ast.proto_fn; }
+        case (token.PRED) { p.bump(); ret ast.proto_fn; }
         case (?t) { unexpected(p, t); }
     }
     fail;
@@ -1767,7 +1768,7 @@ fn parse_ty_params(parser p) -> vec[ast.ty_param] {
     ret ty_params;
 }
 
-fn parse_fn_decl(parser p) -> ast.fn_decl {
+fn parse_fn_decl(parser p, ast.purity purity) -> ast.fn_decl {
     auto pf = parse_arg;
     let util.common.spanned[vec[ast.arg]] inputs =
         // FIXME: passing parse_arg as an lval doesn't work at the
@@ -1790,11 +1791,12 @@ fn parse_fn_decl(parser p) -> ast.fn_decl {
     } else {
         output = @spanned(inputs.span.lo, inputs.span.hi, ast.ty_nil);
     }
-    ret rec(inputs=inputs.node, output=output);
+    // FIXME
+    ret rec(inputs=inputs.node, output=output, purity=purity);
 }
 
-fn parse_fn(parser p, ast.proto proto) -> ast._fn {
-    auto decl = parse_fn_decl(p);
+fn parse_fn(parser p, ast.proto proto, ast.purity purity) -> ast._fn {
+    auto decl = parse_fn_decl(p, purity);
     auto body = parse_block(p);
     ret rec(decl = decl,
             proto = proto,
@@ -1808,11 +1810,11 @@ fn parse_fn_header(parser p)
     ret tup(id, ty_params);
 }
 
-fn parse_item_fn_or_iter(parser p) -> @ast.item {
+fn parse_item_fn_or_iter(parser p, ast.purity purity) -> @ast.item {
     auto lo = p.get_lo_pos();
     auto proto = parse_proto(p);
     auto t = parse_fn_header(p);
-    auto f = parse_fn(p, proto);
+    auto f = parse_fn(p, proto, purity);
     auto item = ast.item_fn(t._0, f, t._1,
                             p.next_def_id(), ast.ann_none);
     ret @spanned(lo, f.body.span.hi, item);
@@ -1830,7 +1832,7 @@ fn parse_method(parser p) -> @ast.method {
     auto lo = p.get_lo_pos();
     auto proto = parse_proto(p);
     auto ident = parse_ident(p);
-    auto f = parse_fn(p, proto);
+    auto f = parse_fn(p, proto, ast.impure_fn);
     auto meth = rec(ident=ident, meth=f,
                     id=p.next_def_id(), ann=ast.ann_none);
     ret @spanned(lo, f.body.span.hi, meth);
@@ -1843,7 +1845,8 @@ fn parse_dtor(parser p) -> @ast.method {
     let vec[ast.arg] inputs = vec();
     let @ast.ty output = @spanned(lo, lo, ast.ty_nil);
     let ast.fn_decl d = rec(inputs=inputs,
-                            output=output);
+                            output=output,
+                            purity=ast.impure_fn);
     let ast._fn f = rec(decl = d,
                         proto = ast.proto_fn,
                         body = b);
@@ -1946,7 +1949,7 @@ fn parse_item_native_fn(parser p) -> @ast.native_item {
     auto lo = p.get_lo_pos();
     expect(p, token.FN);
     auto t = parse_fn_header(p);
-    auto decl = parse_fn_decl(p);
+    auto decl = parse_fn_decl(p, ast.impure_fn);
     auto link_name = none[str];
     if (p.peek() == token.EQ) {
         p.bump();
@@ -2155,6 +2158,7 @@ fn peeking_at_item(parser p) -> bool {
         case (token.GC) { ret true; }
         case (token.CONST) { ret true; }
         case (token.FN) { ret true; }
+        case (token.PRED) { ret true; }
         case (token.ITER) { ret true; }
         case (token.MOD) { ret true; }
         case (token.TYPE) { ret true; }
@@ -2176,11 +2180,17 @@ fn parse_item(parser p) -> @ast.item {
 
         case (token.FN) {
             assert (lyr == ast.layer_value);
-            ret parse_item_fn_or_iter(p);
+            ret parse_item_fn_or_iter(p, ast.impure_fn);
         }
+
+        case (token.PRED) {
+            assert (lyr == ast.layer_value);
+            ret parse_item_fn_or_iter(p, ast.pure_fn);
+        }
+
         case (token.ITER) {
             assert (lyr == ast.layer_value);
-            ret parse_item_fn_or_iter(p);
+            ret parse_item_fn_or_iter(p, ast.impure_fn);
         }
         case (token.MOD) {
             assert (lyr == ast.layer_value);
diff --git a/src/comp/front/token.rs b/src/comp/front/token.rs
index 594515f2..b0fa0588 100644
--- a/src/comp/front/token.rs
+++ b/src/comp/front/token.rs
@@ -156,6 +156,7 @@ tag token {
 
     /* Callable type constructors */
     FN;
+    PRED;
     ITER;
 
     /* Object type and related keywords */
@@ -340,6 +341,7 @@ fn to_str(token t) -> str {
 
         /* Callable type constructors */
         case (FN) { ret "fn"; }
+        case (PRED) { ret "pred"; }
         case (ITER) { ret "iter"; }
 
         /* Object type */
diff --git a/src/comp/middle/fold.rs b/src/comp/middle/fold.rs
index 49b6290c..b37f7be8 100644
--- a/src/comp/middle/fold.rs
+++ b/src/comp/middle/fold.rs
@@ -30,6 +30,7 @@ import front.ast.def;
 import front.ast.def_id;
 import front.ast.ann;
 import front.ast.mt;
+import front.ast.purity;
 
 import std._uint;
 import std._vec;
@@ -301,7 +302,8 @@ type ast_fold[ENV] =
 
      (fn(&ENV e,
          vec[arg] inputs,
-         @ty output) -> ast.fn_decl)              fold_fn_decl,
+         @ty output,
+         purity p) -> ast.fn_decl)                fold_fn_decl,
 
      (fn(&ENV e, &ast._mod m) -> ast._mod)        fold_mod,
 
@@ -900,7 +902,7 @@ fn fold_fn_decl[ENV](&ENV env, ast_fold[ENV] fld,
         inputs += vec(fold_arg(env, fld, a));
     }
     auto output = fold_ty[ENV](env, fld, decl.output);
-    ret fld.fold_fn_decl(env, inputs, output);
+    ret fld.fold_fn_decl(env, inputs, output, decl.purity);
 }
 
 fn fold_fn[ENV](&ENV env, ast_fold[ENV] fld, &ast._fn f) -> ast._fn {
@@ -1542,8 +1544,9 @@ fn identity_fold_block[ENV](&ENV e, &span sp, &ast.block_ blk) -> block {
 
 fn identity_fold_fn_decl[ENV](&ENV e,
                               vec[arg] inputs,
-                              @ty output) -> ast.fn_decl {
-    ret rec(inputs=inputs, output=output);
+                              @ty output,
+                              purity p) -> ast.fn_decl {
+    ret rec(inputs=inputs, output=output, purity=p);
 }
 
 fn identity_fold_fn[ENV](&ENV e,
@@ -1732,7 +1735,7 @@ fn new_identity_fold[ENV]() -> ast_fold[ENV] {
          fold_ann = bind identity_fold_ann[ENV](_,_),
 
          fold_fn = bind identity_fold_fn[ENV](_,_,_,_),
-         fold_fn_decl = bind identity_fold_fn_decl[ENV](_,_,_),
+         fold_fn_decl = bind identity_fold_fn_decl[ENV](_,_,_,_),
          fold_mod = bind identity_fold_mod[ENV](_,_),
          fold_native_mod = bind identity_fold_native_mod[ENV](_,_),
          fold_crate = bind identity_fold_crate[ENV](_,_,_,_),
diff --git a/src/comp/middle/typeck.rs b/src/comp/middle/typeck.rs
index e693fdf7..4cea077f 100644
--- a/src/comp/middle/typeck.rs
+++ b/src/comp/middle/typeck.rs
@@ -8,7 +8,7 @@ import driver.session;
 import util.common;
 import util.common.span;
 import util.common.plain_ann;
-
+import util.common.new_def_hash;
 import util.common.log_expr_err;
 
 import middle.ty;
@@ -40,6 +40,7 @@ import std.map.hashmap;
 import std.option;
 import std.option.none;
 import std.option.some;
+import std.option.from_maybe;
 
 import pretty.pprust;
 
@@ -53,6 +54,7 @@ tag any_item {
 }
 
 type ty_item_table = hashmap[ast.def_id,any_item];
+type fn_purity_table = hashmap[ast.def_id, ast.purity];
 
 type unify_cache_entry = tup(ty.t,ty.t,vec[mutable ty.t]);
 type unify_cache = hashmap[unify_cache_entry,ty.Unify.result];
@@ -62,6 +64,7 @@ type crate_ctxt = rec(session.session sess,
                       @ty_item_table item_items,
                       vec[ast.obj_field] obj_fields,
                       option.t[ast.def_id] this_obj,
+                      @fn_purity_table fn_purity_table,
                       mutable int next_var_id,
                       unify_cache unify_cache,
                       mutable uint cache_hits,
@@ -69,6 +72,7 @@ type crate_ctxt = rec(session.session sess,
                       ty.ctxt tcx);
 
 type fn_ctxt = rec(ty.t ret_ty,
+                   ast.purity purity,
                    @ty_table locals,
                    @crate_ctxt ccx);
 
@@ -1673,6 +1677,62 @@ fn check_pat(&@fn_ctxt fcx, @ast.pat pat) -> @ast.pat {
     ret @fold.respan[ast.pat_](pat.span, new_pat);
 }
 
+fn require_impure(&session.session sess,
+      &ast.purity f_purity, &span sp) -> () {
+    alt (f_purity) {
+        case (ast.impure_fn) {
+            ret;
+        }
+        case (ast.pure_fn) {
+            sess.span_err(sp,
+               "Found impure expression in pure function decl");
+        }
+    }
+}
+
+fn get_function_purity(@crate_ctxt ccx, &ast.def_id d_id) -> ast.purity {
+    let option.t[ast.purity] o = ccx.fn_purity_table.find(d_id);
+    ret from_maybe[ast.purity](ast.impure_fn, o);
+}
+
+fn require_pure_call(@crate_ctxt ccx,
+    &ast.purity caller_purity, @ast.expr callee, &span sp) -> () {
+    alt (caller_purity) {
+        case (ast.impure_fn) {
+            ret;
+        }
+        case (ast.pure_fn) {
+            alt (callee.node) {
+                case (ast.expr_path(_, some[ast.def](ast.def_fn(?d_id)), _)) {
+                    alt (get_function_purity(ccx, d_id)) {
+                            case (ast.pure_fn) {
+                                ret;
+                            }
+                            case (_) {
+                                ccx.sess.span_err(sp,
+                                  "Pure function calls impure function");
+
+                            }
+                        }
+                }
+                case (_) {
+                    ccx.sess.span_err(sp,
+                      "Pure function calls unknown function");
+                }
+            }
+        }
+    }
+}
+
+fn require_pure_function(@crate_ctxt ccx, &ast.def_id d_id, &span sp) -> () {
+    alt (get_function_purity(ccx, d_id)) {
+        case (ast.impure_fn) {
+            ccx.sess.span_err(sp, "Found non-predicate in check expression");
+        }
+        case (_) { ret; }
+    }
+}
+
 fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
     //fcx.ccx.sess.span_warn(expr.span, "typechecking expr " +
     //                       pretty.pprust.expr_to_str(expr));
@@ -1916,6 +1976,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_put(?expr_opt, _)) {
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             alt (expr_opt) {
                 case (none[@ast.expr]) {
                     auto nil = ty.mk_nil(fcx.ccx.tcx);
@@ -1965,7 +2027,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
             alt (e.node) {
                 case (ast.expr_call(?operator, ?operands, _)) {
                     alt (operator.node) {
-                        case (ast.expr_path(?oper_name, ?d_id, _)) {
+                        case (ast.expr_path(?oper_name,
+                                some[ast.def](ast.def_fn(?d_id)), _)) {
 
                             for (@ast.expr operand in operands) {
                                 if (! ast.is_constraint_arg(operand)) {
@@ -1975,8 +2038,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
                                 }
                             }
                             
-                         /* operator must be a pure function */
-                         /* FIXME: need more checking */
+                            require_pure_function(fcx.ccx, d_id, expr.span);
+
                             ret @fold.respan[ast.expr_]
                                 (expr.span, ast.expr_check(expr_t,
                                    plain_ann(fcx.ccx.tcx)));
@@ -2005,6 +2068,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_assign(?lhs, ?rhs, _)) {
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             auto checked = check_assignment(fcx, lhs, rhs);
             auto newexpr = ast.expr_assign(checked._0,
                                            checked._1,
@@ -2013,6 +2078,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_assign_op(?op, ?lhs, ?rhs, _)) {
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             auto checked = check_assignment(fcx, lhs, rhs);
             auto newexpr = ast.expr_assign_op(op,
                                               checked._0,
@@ -2022,6 +2089,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_send(?lhs, ?rhs, _)) {
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             auto lhs_0 = check_expr(fcx, lhs);
             auto rhs_0 = check_expr(fcx, rhs);
             auto rhs_t = expr_ty(fcx.ccx.tcx, rhs_0);
@@ -2045,6 +2114,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_recv(?lhs, ?rhs, _)) {
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             auto lhs_0 = check_expr(fcx, lhs);
             auto rhs_0 = check_expr(fcx, rhs);
             auto lhs_t1 = expr_ty(fcx.ccx.tcx, lhs_0);
@@ -2250,6 +2321,12 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_call(?f, ?args, _)) {
+            /* here we're kind of hosed, as f can be any expr
+             need to restrict it to being an explicit expr_path if we're
+            inside a pure function, and need an environment mapping from 
+            function name onto purity-designation */
+            require_pure_call(fcx.ccx, fcx.purity, f, expr.span);
+
             auto result = check_call(fcx, f, args);
             auto f_1 = result._0;
             auto args_1 = result._1;
@@ -2302,6 +2379,8 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
 
             auto ann = triv_ann(t);
 
+            require_impure(fcx.ccx.sess, fcx.purity, expr.span);
+
             ret @fold.respan[ast.expr_](expr.span,
                                         ast.expr_self_method(id, ann));
         }
@@ -2725,6 +2804,7 @@ fn check_const(&@crate_ctxt ccx, &span sp, ast.ident ident, @ast.ty t,
     // for checking the initializer expression.
     auto rty = ann_to_type(ann);
     let @fn_ctxt fcx = @rec(ret_ty = rty,
+                            purity = ast.pure_fn,
                             locals = @common.new_def_hash[ty.t](),
                             ccx = ccx);
     auto e_ = check_expr(fcx, e);
@@ -2755,6 +2835,7 @@ fn check_fn(&@crate_ctxt ccx, &ast.fn_decl decl, ast.proto proto,
     }
 
     let @fn_ctxt fcx = @rec(ret_ty = ast_ty_to_ty_crate(ccx, decl.output),
+                            purity = decl.purity,
                             locals = local_ty_table,
                             ccx = ccx);
 
@@ -2834,6 +2915,26 @@ fn eq_unify_cache_entry(&unify_cache_entry a, &unify_cache_entry b) -> bool {
     ret true;
 }
 
+fn mk_fn_purity_table(@ast.crate crate) -> @fn_purity_table {
+    auto res = @new_def_hash[ast.purity]();
+
+    fn do_one(@fn_purity_table t, @ast.item i) -> () {
+        alt (i.node) {
+            case (ast.item_fn(_, ?f, _, ?d_id, _)) {
+                t.insert(d_id, f.decl.purity);
+            }
+            case (_) {}
+        }
+    }
+
+    auto do_one_fn = bind do_one(res,_);
+    auto v = walk.default_visitor();
+
+    auto add_fn_entry_visitor = rec(visit_item_post=do_one_fn with v);
+
+    walk.walk_crate(add_fn_entry_visitor, *crate);
+    ret res;
+}
 
 type typecheck_result = tup(@ast.crate, ty.type_cache);
 
@@ -2848,12 +2949,15 @@ fn check_crate(ty.ctxt tcx, @ast.crate crate)
     auto eqer = eq_unify_cache_entry;
     auto unify_cache =
         map.mk_hashmap[unify_cache_entry,ty.Unify.result](hasher, eqer);
+    auto fpt =
+        mk_fn_purity_table(crate); // use a variation on Collect
 
     auto ccx = @rec(sess=sess,
                     type_cache=result._1,
                     item_items=result._2,
                     obj_fields=fields,
                     this_obj=none[ast.def_id],
+                    fn_purity_table = fpt,
                     mutable next_var_id=0,
                     unify_cache=unify_cache,
                     mutable cache_hits=0u,
diff --git a/src/comp/pretty/pprust.rs b/src/comp/pretty/pprust.rs
index 62e6594b..c87a8679 100644
--- a/src/comp/pretty/pprust.rs
+++ b/src/comp/pretty/pprust.rs
@@ -790,7 +790,14 @@ fn print_pat(ps s, &@ast.pat pat) {
 
 fn print_fn(ps s, ast.fn_decl decl, str name,
                    vec[ast.ty_param] typarams) {
-    wrd1(s, "fn");
+    alt (decl.purity) {
+        case (ast.impure_fn) {
+            wrd1(s, "fn");
+        }
+        case (_) {
+            wrd1(s, "pred");
+        }
+    }
     wrd(s.s, name);
     print_type_params(s, typarams);
     popen(s);