[m-rev.] diff: improved error message from typecheck

[Zoltan Somogyi]

compiler/typecheck.m:
	Even if there is more than one type assignment involved in a type error
	for a unification of the shape X = f(Ys), print a focused error message
	that lists the actual/expected types only for the arguments which
	could possibly cause the type mismatch.

There is no existing test affected by this change, and the motivation that
caused me to make the change (a far too big error message after I modified
a compiler module) is too difficult to cut down to test case size. I did verify
that the compiler now generates a far more comprehensible error message in
that case than before.

Zoltan.

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Index: compiler/typecheck.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/typecheck.m,v
retrieving revision 1.352
diff -u -b -r1.352 typecheck.m
--- compiler/typecheck.m	14 May 2004 08:40:28 -0000	1.352
+++ compiler/typecheck.m	20 May 2004 00:09:15 -0000
@@ -2490,11 +2490,20 @@
 
 :- type args_type_assign
 	--->	args(
-			type_assign, 		% Type assignment,
-			list(type), 		% types of callee args,
-			class_constraints	% constraints from callee
+			caller_arg_assign	:: type_assign,
+						% Type assignment,
+			callee_arg_types	:: list(type),
+						% types of callee args,
+			callee_constraints	:: class_constraints
+						% constraints from callee
 		).
 
+:- func get_caller_arg_assign(args_type_assign) = type_assign.
+:- func get_callee_arg_types(args_type_assign) = list(type).
+
+get_caller_arg_assign(ArgsTypeAssign) = ArgsTypeAssign ^ caller_arg_assign.
+get_callee_arg_types(ArgsTypeAssign) = ArgsTypeAssign ^ callee_arg_types.
+
 :- pred typecheck_unify_var_functor_get_ctors(type_assign_set::in,
 	typecheck_info::in, list(cons_type_info)::in,
 	cons_type_assign_set::in, cons_type_assign_set::out) is det.
@@ -4816,30 +4825,40 @@
 	io__write_string("\n"),
 	prog_out__write_context(Context),
 	io__write_string("  and term `"),
-	{ strip_builtin_qualifier_from_cons_id(Functor, Functor1) },
-	hlds_out__write_functor_cons_id(Functor1, Args, VarSet, ModuleInfo, no),
+	{ strip_builtin_qualifier_from_cons_id(Functor, StrippedFunctor) },
+	hlds_out__write_functor_cons_id(StrippedFunctor, Args, VarSet,
+		ModuleInfo, no),
 	io__write_string("':\n"),
 	prog_out__write_context(Context),
 	io__write_string("  type error in argument(s) of "),
-	write_functor_name(Functor1, Arity),
+	write_functor_name(StrippedFunctor, Arity),
 	io__write_string(".\n"),
 
+	{ ConsArgTypesSet = list__map(get_callee_arg_types,
+		ArgsTypeAssignSet) },
+
 	% If we know the type of the function symbol, and each argument
 	% also has at most one possible type, then we prefer to print an
 	% error message that mentions the actual and expected types of the
 	% arguments only for the arguments in which the two types differ.
 	(
-		{ ArgsTypeAssignSet = [SingleArgsTypeAssign] },
-		{ SingleArgsTypeAssign = args(TypeAssign, ConsArgTypes, _) },
+		{ list__all_same(ConsArgTypesSet) },
+		{ ConsArgTypesSet = [ConsArgTypes | _] },
 		{ assoc_list__from_corresponding_lists(Args, ConsArgTypes,
 			ArgExpTypes) },
-		{ find_mismatched_args(ArgExpTypes, [TypeAssign], 1,
-			Mismatches) },
-		{ Mismatches = [_ | _] }
+		{ TypeAssigns = list__map(get_caller_arg_assign,
+			ArgsTypeAssignSet) },
+		{ find_mismatched_args(ArgExpTypes, TypeAssigns, 1,
+			SimpleMismatches, ComplexMismatches, AllMismatches) },
+		{ require(list__is_not_empty(AllMismatches),
+			"report_error_functor_arg_types: no mismatches") },
+		{ ComplexMismatches = [] }
 	->
-		report_mismatched_args(Mismatches, yes, VarSet, Functor,
+		report_mismatched_args(SimpleMismatches, yes, VarSet, Functor,
 			Context)
 	;
+		% XXX If we can compute AllMismatches, then we should use it
+		% to report which arguments are OK, and which are suspect.
 
 		{ convert_args_type_assign_set(ArgsTypeAssignSet,
 			TypeAssignSet) },
@@ -4877,9 +4896,15 @@
 	).
 
 :- type mismatch_info
-	--->	mismatch(
+	--->	mismatch_info(
 			int,		% argument number, starting from 1
 			prog_var,	% variable in that position
+			list(type_mismatch)
+					% list of possible type mismatches
+		).
+
+:- type type_mismatch
+	--->	type_mismatch(
 			type,		% actual type of that variable
 			type,		% expected type of that variable
 			tvarset,	% the type vars in the expected
@@ -4888,15 +4913,43 @@
 		).
 
 :- pred find_mismatched_args(assoc_list(prog_var, type)::in,
-	type_assign_set::in, int::in, list(mismatch_info)::out) is semidet.
+	type_assign_set::in, int::in, list(mismatch_info)::out,
+	list(mismatch_info)::out, list(mismatch_info)::out) is det.
 
-find_mismatched_args([], _, _, []).
+find_mismatched_args([], _, _, [], [], []).
 find_mismatched_args([Arg - ExpType | ArgExpTypes], TypeAssignSet, ArgNum0,
-		Mismatched) :-
+		SimpleMismatches, ComplexMismatches, AllMismatches) :-
 	ArgNum1 = ArgNum0 + 1,
-	find_mismatched_args(ArgExpTypes, TypeAssignSet, ArgNum1, Mismatched1),
+	find_mismatched_args(ArgExpTypes, TypeAssignSet, ArgNum1,
+		SimpleMismatchesTail, ComplexMismatchesTail,
+		AllMismatchesTail),
 	get_type_stuff(TypeAssignSet, Arg, TypeStuffList),
-	TypeStuffList = [TypeStuff],
+	list__filter_map(substitute_types_check_match(ExpType), TypeStuffList,
+		TypeMismatches0),
+	list__sort_and_remove_dups(TypeMismatches0, TypeMismatches),
+	(
+		TypeMismatches = [],
+		SimpleMismatches = SimpleMismatchesTail,
+		ComplexMismatches = ComplexMismatchesTail,
+		AllMismatches = AllMismatchesTail
+	;
+		TypeMismatches = [_],
+		Mismatch = mismatch_info(ArgNum0, Arg, TypeMismatches),
+		SimpleMismatches = [Mismatch | SimpleMismatchesTail],
+		ComplexMismatches = ComplexMismatchesTail,
+		AllMismatches = [Mismatch | AllMismatchesTail]
+	;
+		TypeMismatches = [_, _ | _],
+		Mismatch = mismatch_info(ArgNum0, Arg, TypeMismatches),
+		SimpleMismatches = SimpleMismatchesTail,
+		ComplexMismatches = [Mismatch | ComplexMismatchesTail],
+		AllMismatches = [Mismatch | AllMismatchesTail]
+	).
+
+:- pred substitute_types_check_match((type)::in, type_stuff::in,
+	type_mismatch::out) is semidet.
+
+substitute_types_check_match(ExpType, TypeStuff, TypeMismatch) :-
 	TypeStuff = type_stuff(ArgType, TVarSet, TypeBindings, HeadTypeParams),
 	term__apply_rec_substitution(ArgType, TypeBindings, FullArgType),
 	term__apply_rec_substitution(ExpType, TypeBindings, FullExpType),
@@ -4911,10 +4964,10 @@
 			FullArgType = term__functor(term__atom("<any>"), [], _)
 		)
 	->
-		Mismatched = Mismatched1
+		fail
 	;
-		Mismatched = [mismatch(ArgNum0, Arg, FullArgType, FullExpType,
-			TVarSet, HeadTypeParams) | Mismatched1]
+		TypeMismatch = type_mismatch(FullArgType, FullExpType,
+			TVarSet, HeadTypeParams)
 	).
 
 :- pred report_mismatched_args(list(mismatch_info)::in, bool::in,
@@ -4923,8 +4976,13 @@
 report_mismatched_args([], _, _, _, _) --> [].
 report_mismatched_args([Mismatch | Mismatches], First, VarSet, Functor,
 		Context) -->
-	{ Mismatch = mismatch(ArgNum, Var, ActType, ExpType, TVarSet,
-		HeadTypeParams) },
+	{ Mismatch = mismatch_info(ArgNum, Var, TypeMismatches) },
+	{ TypeMismatches = [TypeMismatch] ->
+		TypeMismatch = type_mismatch(ActType, ExpType, TVarSet,
+			HeadTypeParams)
+	;
+		error("report_mismatched_args: more than one type mismatch")
+	},
 	prog_out__write_context(Context),
 	(
 		% Handle higher-order syntax such as ''(F, A) specially:
@@ -5000,13 +5058,13 @@
 :- pred write_functor_name(cons_id::in, int::in, io::di, io::uo) is det.
 
 write_functor_name(Functor, Arity) -->
-	{ strip_builtin_qualifier_from_cons_id(Functor, Functor1) },
+	{ strip_builtin_qualifier_from_cons_id(Functor, StrippedFunctor) },
 	( { Arity = 0 } ->
 		io__write_string("constant `"),
 		( { Functor = cons(Name, _) } ->
 			prog_out__write_sym_name(Name)
 		;
-			hlds_out__write_cons_id(Functor1)
+			hlds_out__write_cons_id(StrippedFunctor)
 		),
 		io__write_string("'")
 	; { Functor = cons(unqualified(""), _) } ->
@@ -5015,7 +5073,7 @@
 		io__write_string(")")
 	;
 		io__write_string("functor `"),
-		hlds_out__write_cons_id(Functor1),
+		hlds_out__write_cons_id(StrippedFunctor),
 		io__write_string("'")
 	).
 
@@ -5863,8 +5921,8 @@
 		;
 			io__write_string("  error: undefined symbol `"),
 			{ strip_builtin_qualifier_from_cons_id(Functor,
-				Functor1) },
-			hlds_out__write_cons_id(Functor1),
+				StrippedFunctor) },
+			hlds_out__write_cons_id(StrippedFunctor),
 			io__write_string("'"),
 			(
 				{ Functor = cons(Constructor, _) },
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