compiler: forbid cascading match cases

Erlang's `case` expression does not have an equivalent that we can
translate this to idiomatically, so we're forbidding it for now.

We'll revisit later on.

Close #9

src/compiler/ocaml_to_erlang/error.ml

@@ -11,6 +11,20 @@ let ppf = Format.err_formatter
 let file_a_bug =
   {| If you think this is a bug, please file an issue here: https://github.com/AbstractMachinesLab/caramel/issues/new |}
 
+let unsupported_fallthrough_cases () =
+  Format.fprintf ppf
+    {|We have found a case expression that falls through to the next case, like:
+
+  match x with
+  | 0 | 1 -> true   <--- this branch falls through
+  | _ -> false
+
+Since these patterns are not possible in Erlang, Caramel does not support them
+at the moment.
+\n
+|};
+  exit 1
+
 let unsupported_let_rec_inside_of_function_body () =
   Format.fprintf ppf
     {|We have found a let rec binding within a function.

src/compiler/ocaml_to_erlang/fun.ml

@@ -125,6 +125,7 @@ and mk_pattern :
       let value = mk_pattern expr ~var_names in
       Pat.tuple [ tag; value ]
   | Tpat_constant const -> Erlang.Ast.Pattern_match (const_to_literal const)
+  | Tpat_or (_, _, _) -> Error.unsupported_fallthrough_cases ()
   (* NOTE: here's where the translation of pattern
    * matching at the function level should happen. *)
   | _ -> Erlang.Ast.Pattern_ignore

tests/compiler/expressions.t/match.ml

@@ -37,8 +37,3 @@ let match_tuples () =
 
 let match_atoms () =
   match `Hello with `Xavier -> true | `Joe -> true | _ -> false
-
-let match_fall_through () =
-  match 0 with
-  | 1 | 2 -> true
-  | _ -> false

tests/compiler/expressions.t/match_fallthrough.ml

@@ -0,0 +1,4 @@
+let match_fall_through () =
+  match 0 with
+  | 1 | 2 -> true
+  | _ -> false

tests/compiler/expressions.t/run.t

@@ -8,6 +8,7 @@
   list.ml
   literals.ml
   match.ml
+  match_fallthrough.ml
   names.ml
   names_primes.ml
   record_update.ml
@@ -381,7 +382,6 @@
   -export_type([int_pair/0]).
 
   -export([match_atoms/0]).
-  -export([match_fall_through/0]).
   -export([match_ignore/0]).
   -export([match_int/0]).
   -export([match_list/0]).
@@ -459,13 +459,6 @@
       _ -> false
     end.
 
-  -spec match_fall_through() -> boolean().
-  match_fall_through() ->
-    case 0 of
-      _ -> true;
-      _ -> false
-    end.
-
 
   $ caramelc compile names.ml
   File "names.ml", line 15, characters 2-13:
@@ -629,3 +622,17 @@
      }.
 
 
+  $ caramelc compile match_fallthrough.ml
+  We have found a case expression that falls through to the next case, like:
+
+    match x with
+    | 0 | 1 -> true   <--- this branch falls through
+    | _ -> false
+
+  Since these patterns are not possible in Erlang, Caramel does not support them
+  at the moment.
+  \n
+  [1]
+  $ cat match_fallthrough.erl
+  cat: match_fallthrough.erl: No such file or directory
+  [1]