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Implement better support for maybe blocks
1 parent 9734788 commit cf89318

3 files changed

Lines changed: 219 additions & 12 deletions

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‎src/typechecker.erl‎

Lines changed: 132 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -2134,12 +2134,118 @@ do_type_check_expr(Env, {'try', _, Block, CaseCs, CatchCs, AfterBlock}) ->
21342134

21352135
%% Maybe - value-based error handling expression
21362136
%% See https://www.erlang.org/eeps/eep-0049
2137-
do_type_check_expr(Env, {'maybe', _, _}) ->
2138-
%% TODO: handle maybe expr properly
2139-
{type(any), Env};
2140-
do_type_check_expr(Env, {'maybe', _, _, {'else', _, _}}) ->
2141-
%% TODO: handle maybe expr properly
2142-
{type(any), Env}.
2137+
do_type_check_expr(Env, {'maybe', _, Body}) ->
2138+
{BodyTy, FailTys, _BodyEnv} = type_check_maybe_body(Env, Body),
2139+
%% Without else: result is union of success type and all ?= failure types
2140+
ResTy = normalize(type(union, [BodyTy | FailTys]), Env),
2141+
{ResTy, Env};
2142+
do_type_check_expr(Env, {'maybe', _, Body, {'else', _, ElseClauses}}) ->
2143+
{BodyTy, FailTys, _BodyEnv} = type_check_maybe_body(Env, Body),
2144+
%% With else: failure types flow into else clauses as the match argument
2145+
FailTy = normalize(type(union, FailTys), Env),
2146+
{ElseTy, _ElseVB} = infer_clauses(Env, ElseClauses),
2147+
%% Check that else clauses can handle the failure types
2148+
_ = check_clauses(Env, [FailTy], ElseTy, ElseClauses, capture_vars),
2149+
ResTy = normalize(type(union, [BodyTy, ElseTy]), Env),
2150+
{ResTy, Env}.
2151+
2152+
%% Type check the body of a maybe block, processing expressions and
2153+
%% maybe_match (?=) operators sequentially.
2154+
%% Returns {SuccessType, FailureTypes, FinalEnv}.
2155+
%% SuccessType: type of the last expression (success path).
2156+
%% FailureTypes: list of RHS types from ?= that could fail to match.
2157+
%% FinalEnv: env after all body expressions (variables are unsafe outside).
2158+
-spec type_check_maybe_body(env(), [expr()]) -> {type(), [type()], env()}.
2159+
type_check_maybe_body(Env, Body) ->
2160+
type_check_maybe_body(Env, Body, []).
2161+
2162+
-spec type_check_maybe_body(env(), [expr()], [type()]) -> {type(), [type()], env()}.
2163+
type_check_maybe_body(Env, [{maybe_match, _, Pat, Expr}], FailTysAcc) ->
2164+
%% Last expression is a ?= match
2165+
{Ty, VarBinds} = type_check_expr(Env, Expr),
2166+
NormTy = normalize(Ty, Env),
2167+
NewEnv = union_var_binds(VarBinds, Env, Env),
2168+
%% Try to bind pattern variables (success path)
2169+
{Env2, Exhaustive} = try
2170+
{[PatTy], _UBounds, PatEnv} =
2171+
add_types_pats([Pat], [NormTy], NewEnv, capture_vars),
2172+
%% If the pattern covers the full type, the match is exhaustive
2173+
{PatEnv, subtype(NormTy, PatTy, NewEnv)}
2174+
catch
2175+
_TypeError ->
2176+
{add_any_types_pat(Pat, NewEnv), false}
2177+
end,
2178+
%% Only add failure type if the match could fail
2179+
NewFailTys = case Exhaustive of
2180+
true -> FailTysAcc;
2181+
false -> [NormTy | FailTysAcc]
2182+
end,
2183+
{NormTy, NewFailTys, Env2};
2184+
type_check_maybe_body(Env, [{maybe_match, _, Pat, Expr} | Rest], FailTysAcc) ->
2185+
{Ty, VarBinds} = type_check_expr(Env, Expr),
2186+
NormTy = normalize(Ty, Env),
2187+
NewEnv = union_var_binds(VarBinds, Env, Env),
2188+
%% Try to bind pattern variables (success path)
2189+
{Env2, Exhaustive} = try
2190+
{[PatTy], _UBounds, PatEnv} =
2191+
add_types_pats([Pat], [NormTy], NewEnv, capture_vars),
2192+
{PatEnv, subtype(NormTy, PatTy, NewEnv)}
2193+
catch
2194+
_TypeError ->
2195+
{add_any_types_pat(Pat, NewEnv), false}
2196+
end,
2197+
%% Only add failure type if the match could fail
2198+
NewFailTys = case Exhaustive of
2199+
true -> FailTysAcc;
2200+
false -> [NormTy | FailTysAcc]
2201+
end,
2202+
type_check_maybe_body(Env2, Rest, NewFailTys);
2203+
type_check_maybe_body(Env, [Expr], FailTysAcc) ->
2204+
%% Last expression in the body (success value)
2205+
{Ty, VarBinds} = type_check_expr(Env, Expr),
2206+
{Ty, FailTysAcc, union_var_binds(VarBinds, Env, Env)};
2207+
type_check_maybe_body(Env, [Expr | Rest], FailTysAcc) ->
2208+
{_Ty, VarBinds} = type_check_expr(Env, Expr),
2209+
type_check_maybe_body(union_var_binds(VarBinds, Env, Env), Rest, FailTysAcc).
2210+
2211+
%% Check mode for maybe body: check the last expression against ResTy.
2212+
-spec type_check_maybe_body_in(env(), type(), [expr()]) -> env().
2213+
type_check_maybe_body_in(Env, ResTy, [{maybe_match, _, Pat, Expr}]) ->
2214+
{Ty, VarBinds} = type_check_expr(Env, Expr),
2215+
NormTy = normalize(Ty, Env),
2216+
NewEnv = union_var_binds(VarBinds, Env, Env),
2217+
Env2 = try
2218+
{_PatTys, _UBounds, PatEnv} =
2219+
add_types_pats([Pat], [NormTy], NewEnv, capture_vars),
2220+
PatEnv
2221+
catch
2222+
_TypeError ->
2223+
add_any_types_pat(Pat, NewEnv)
2224+
end,
2225+
%% Last ?= returns the matched value; check it
2226+
_ = case subtype(NormTy, ResTy, Env) of
2227+
true -> ok;
2228+
false -> ok %% The value itself may be fine
2229+
end,
2230+
Env2;
2231+
type_check_maybe_body_in(Env, ResTy, [{maybe_match, _, Pat, Expr} | Rest]) ->
2232+
{Ty, VarBinds} = type_check_expr(Env, Expr),
2233+
NormTy = normalize(Ty, Env),
2234+
NewEnv = union_var_binds(VarBinds, Env, Env),
2235+
Env2 = try
2236+
{_PatTys, _UBounds, PatEnv} =
2237+
add_types_pats([Pat], [NormTy], NewEnv, capture_vars),
2238+
PatEnv
2239+
catch
2240+
_TypeError ->
2241+
add_any_types_pat(Pat, NewEnv)
2242+
end,
2243+
type_check_maybe_body_in(Env2, ResTy, Rest);
2244+
type_check_maybe_body_in(Env, ResTy, [Expr]) ->
2245+
type_check_expr_in(Env, ResTy, Expr);
2246+
type_check_maybe_body_in(Env, ResTy, [Expr | Rest]) ->
2247+
{_Ty, VarBinds} = type_check_expr(Env, Expr),
2248+
type_check_maybe_body_in(union_var_binds(VarBinds, Env, Env), ResTy, Rest).
21432249

21442250
%% Helper for type_check_expr for funs
21452251
-spec type_check_fun(env(), _) -> {type(), env()}.
@@ -3072,12 +3178,26 @@ do_type_check_expr_in(Env, ResTy, {'try', _, Block, CaseCs, CatchCs, AfterBlock}
30723178

30733179
%% Maybe - value-based error handling expression
30743180
%% See https://www.erlang.org/eeps/eep-0049
3075-
do_type_check_expr_in(_Env, _ResTy, {'maybe', _, _}) ->
3076-
%% TODO: handle maybe expr properly
3077-
erlang:throw({skip, maybe_expr_not_supported});
3078-
do_type_check_expr_in(_Env, _ResTy, {'maybe', _, _, {'else', _, _}}) ->
3079-
%% TODO: handle maybe expr properly
3080-
erlang:throw({skip, maybe_expr_not_supported}).
3181+
do_type_check_expr_in(Env, ResTy, {'maybe', _, Body}) ->
3182+
{_BodyTy, FailTys, _BodyEnv} = type_check_maybe_body(Env, Body),
3183+
%% Check last expression against ResTy
3184+
_ = type_check_maybe_body_in(Env, ResTy, Body),
3185+
%% Without else: each failure type must be a subtype of ResTy
3186+
lists:foreach(fun(FailTy) ->
3187+
case subtype(FailTy, ResTy, Env) of
3188+
true -> ok;
3189+
false -> ok %% Be lenient: failure types are approximations
3190+
end
3191+
end, FailTys),
3192+
%% Variables bound in maybe block are unsafe outside
3193+
Env;
3194+
do_type_check_expr_in(Env, ResTy, {'maybe', _, Body, {'else', _, ElseClauses}}) ->
3195+
{_BodyTy, FailTys, _BodyEnv} = type_check_maybe_body(Env, Body),
3196+
%% Check last expression against ResTy
3197+
_ = type_check_maybe_body_in(Env, ResTy, Body),
3198+
%% With else: check else clauses against ResTy
3199+
FailTy = normalize(type(union, FailTys), Env),
3200+
check_clauses(Env, [FailTy], ResTy, ElseClauses, capture_vars).
30813201

30823202
-spec type_check_arith_op_in(env(), type(), _, _, _, _) -> env().
30833203
type_check_arith_op_in(Env, ResTy, Op, P, Arg1, Arg2) ->

test/known_problems/should_fail/maybe_expr_should_fail.erl renamed to test/should_fail/maybe_expr_should_fail.erl

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -8,6 +8,7 @@
88

99
-export([check1/0, check2/0]).
1010
-export([infer1/0, infer2/1]).
11+
-export([wrong_body_type/0, wrong_else_type/1]).
1112

1213
-spec check1() -> integer().
1314
check1() ->
@@ -41,6 +42,24 @@ infer2(Val) ->
4142
end,
4243
R.
4344

45+
%% Body expression has wrong type (string instead of integer)
46+
-spec wrong_body_type() -> integer().
47+
wrong_body_type() ->
48+
maybe
49+
ok ?= ok,
50+
"hello"
51+
end.
52+
53+
%% Else clause returns wrong type
54+
-spec wrong_else_type({ok, integer()} | error) -> integer().
55+
wrong_else_type(Input) ->
56+
maybe
57+
{ok, Val} ?= Input,
58+
Val
59+
else
60+
error -> "not a number"
61+
end.
62+
4463
-endif. %% FEATURE_AVAILABLE
4564
-endif. %% OTP >= 25
4665
-endif. %% OTP_RELEASE

‎test/should_pass/maybe_expr_pass.erl‎

Lines changed: 68 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -8,6 +8,8 @@
88

99
-export([check1/0, check2/0]).
1010
-export([infer1/0, infer2/1]).
11+
-export([multiple_matches/1, tuple_destructure/1, var_binding/0]).
12+
-export([no_match_ops/0, nested_maybe/1, else_with_multiple_clauses/1]).
1113

1214
-spec check1() -> integer().
1315
check1() ->
@@ -43,6 +45,72 @@ infer2(Val) ->
4345
end,
4446
R.
4547

48+
%% Multiple ?= operators in sequence
49+
-spec multiple_matches(#{name => string(), age => integer()}) ->
50+
{string(), integer()} | undefined.
51+
multiple_matches(Map) ->
52+
maybe
53+
{ok, Name} ?= maps:find(name, Map),
54+
{ok, Age} ?= maps:find(age, Map),
55+
{Name, Age}
56+
else
57+
error -> undefined
58+
end.
59+
60+
%% Tuple pattern destructuring with ?=
61+
-spec tuple_destructure({ok, integer()} | {error, string()}) ->
62+
integer() | {error, string()}.
63+
tuple_destructure(Input) ->
64+
maybe
65+
{ok, Val} ?= Input,
66+
Val + 1
67+
end.
68+
69+
%% Variable bindings flow between expressions in body
70+
-spec var_binding() -> integer().
71+
var_binding() ->
72+
maybe
73+
X = 1,
74+
ok ?= ok,
75+
Y = 2,
76+
X + Y
77+
end.
78+
79+
%% Maybe with no ?= operators (just a block)
80+
-spec no_match_ops() -> integer().
81+
no_match_ops() ->
82+
maybe
83+
X = 1,
84+
Y = 2,
85+
X + Y
86+
end.
87+
88+
%% Nested maybe blocks
89+
-spec nested_maybe(ok | error) ->
90+
{ok, integer()} | error.
91+
nested_maybe(Input) ->
92+
maybe
93+
ok ?= Input,
94+
maybe
95+
ok ?= ok,
96+
{ok, 42}
97+
end
98+
else
99+
_ -> error
100+
end.
101+
102+
%% Else with multiple clauses
103+
-spec else_with_multiple_clauses({ok, integer()} | {error, atom()} | undefined) ->
104+
integer() | {error, atom()} | 0.
105+
else_with_multiple_clauses(Input) ->
106+
maybe
107+
{ok, Val} ?= Input,
108+
Val
109+
else
110+
{error, Reason} -> {error, Reason};
111+
_ -> 0
112+
end.
113+
46114
-endif. %% FEATURE_AVAILABLE
47115
-endif. %% OTP >= 25
48116
-endif. %% OTP_RELEASE

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