Improve the error handling situation (#71)

* Sketches of a condition system

* `Condition.condition` -> `Condition.t`

* Migrate `NT` over

* Migrate `Branch` over

* Migrate `Sublanguage` et al. over

* Migrate the PRL over

* Migrate the DRL over

* Migrate the DML over

* Have `Qualified_name.try_parse` return a `Condition.t`

* Migrate the DDL over

* Migrate the VCL over

* Migrate the SCL over

* Migrate the ICL over

* Migrate the parsers over

* Just inherit the errors from NT and the sublanguages

* Migrate `Management.Physical` over

* Migrate `Configuration` over

* Migrate `Management.Branch` over

* Migrate `Transport` over

* Migrate `System` over

* Migrate `Server` over

* Include messages on on all conditions

* A prettier stringifier

2 files changed, 10 insertions, 18 deletions

diff --git a/lib/vcl/executor.ml b/lib/vcl/executor.ml
index 5f3dd49..4ac5eff 100644
--- a/lib/vcl/executor.ml
+++ b/lib/vcl/executor.ml
@@ -1,21 +1,11 @@
 module Make (NT : Nt.S) = struct
-  type error =
-    | ParseError of string
-    | NtError of Nt.error
-
-  let sexp_of_error e =
-    let open Sexplib.Sexp in
-    match e with
-    | ParseError s -> List [ Atom "parse-error"; Atom s ]
-    | NtError e    -> List [ Atom "nt-error"; Atom (Nt.string_of_error e) ]
-
   let execute (ctx : Sublanguage_context.t) (stmt : Ast.statement) :
-      (Sublanguage_types.result, error) result =
+      (Sublanguage_types.result, Condition.t) result =
     match stmt with
     | Ast.Use branch_name ->
-        match ctx.switch_branch branch_name with
-        | Ok delta     -> Ok (Sublanguage_types.Transition delta)
-        | Error e      -> Error (NtError e)
+       Result.map
+         (fun delta -> Sublanguage_types.Transition delta)
+         (ctx.switch_branch branch_name)
 
   let _ = (module NT : Nt.S)
 end
diff --git a/lib/vcl/sublanguage.ml b/lib/vcl/sublanguage.ml
index f9ca629..4319eb8 100644
--- a/lib/vcl/sublanguage.ml
+++ b/lib/vcl/sublanguage.ml
@@ -1,9 +1,13 @@
 module Make (NT : Nt.S) = struct
+  module Error = struct
+    open Condition
+    let unrecognized_command expr = condition "unrecognized-command" "Unrecognized command" ("expression" |=| (of_sexp expr))
+  end
+
   module Exec = Executor.Make (NT)
 
   type configuration = unit
   type ast = Ast.statement
-  type error = Exec.error
 
   let name = "vcl"
   let parse _ = Ok ()
@@ -11,9 +15,7 @@ module Make (NT : Nt.S) = struct
   let parse_sexp = function
     | Sexplib.Sexp.(List [ Atom "use"; Atom branch ]) -> Ok (Ast.Use branch)
     | sexp ->
-        Error (Exec.ParseError ("unrecognized vcl command: " ^ Sexplib.Sexp.to_string sexp))
+        Error (Error.unrecognized_command sexp)
 
   let execute = Exec.execute
-
-  let sexp_of_error = Exec.sexp_of_error
 end