better errors

flap/src/hopix/hopixTypechecker.ml

@@ -126,7 +126,7 @@ and synth_apply
     let given = synth_expression tenv x in
     check_equal_types x.position ~expected ~given;
     droit
-  | _ -> failwith "Error: apply"
+  | _ -> HopixTypes.type_error f.position "Can't apply"
 
 and synth_record
   :  HopixTypes.typing_environment
@@ -167,7 +167,7 @@ and synth_sequence
   =
   fun tenv elist ->
   match elist with
-  | [] -> failwith "Error: sequence"
+  | [] -> HopixTypes.type_error Position.dummy "Invalid sequence"
   | [ x ] -> synth_expression tenv x
   | x :: l ->
     let given = synth_expression tenv x in
@@ -261,10 +261,7 @@ and check_expression
   :  HopixTypes.typing_environment -> HopixAST.expression Position.located
   -> HopixTypes.aty -> unit
   =
-  fun env (Position.{ value = e; position = pos } as exp) expected ->
-  match e with
-  | Fun df -> failwith "Students! This is your job! (check_expression | Fun)"
-  | _ ->
+  fun env (Position.{ position = pos } as exp) expected ->
   let given = synth_expression env exp in
   check_equal_types pos ~expected ~given
 
@@ -275,7 +272,7 @@ and check_value_definition
   fun env -> function
   | SimpleValue (id, ty, ex) ->
     (match ty with
-     | None -> failwith "A type is missing."
+     | None -> HopixTypes.type_error ex.position "Type is missing"
      | Some ty' ->
        let tys, tenv = Position.located_pos (check_type_scheme env) ty' in
        check_expression tenv ex (HopixTypes.instantiate_type_scheme tys []);