Suggest most similar function id when we can't find a function type

Find identifier with smallest optimal string alignment (OSA) distance
(within reason) and suggest it. The max difference is the identifier
length - 3 clamped between 1 and 4. So a 4 character identifier will
only suggest 1 change edits, a 5 character identifier 2 character edits
and so on up to 4 edits.

src/lib/type_env.ml

@@ -898,7 +898,7 @@ let add_typ_synonym id typq arg env =
 
 let get_val_spec_orig id env =
   try get_item (id_loc id) env (Bindings.find id env.global.val_specs)
-  with Not_found -> typ_error (id_loc id) ("No type signature found for " ^ string_of_id id)
+  with Not_found -> typ_error (id_loc id) ("No function type found for " ^ string_of_id id)
 
 let get_val_spec_opt id env =
   match Bindings.find_opt id env.global.val_specs with
@@ -922,7 +922,22 @@ let get_val_spec_opt id env =
 let get_val_spec id env =
   match get_val_spec_opt id env with
   | Some (bind, _) -> bind
-  | None -> typ_error (id_loc id) ("No type declaration found for " ^ string_of_id id)
+  | None ->
+      (* Try to find the most similar function name, within reason, to include in the error *)
+      let closest = ref (Int.max_int, None) in
+      Bindings.iter
+        (fun other_id item ->
+          let id_str = string_of_id id in
+          let other_str = string_of_id other_id in
+          if abs (String.length id_str - String.length other_str) <= 2 then (
+            let distance = Util.levenshtein_distance ~osa:true id_str other_str in
+            let max_distance = min 4 (max 1 (String.length id_str - 3)) in
+            if distance <= max_distance && distance < fst !closest then closest := (distance, Some other_str)
+          )
+        )
+        env.global.val_specs;
+      let hint_msg = match snd !closest with Some other_str -> "\n\nDid you mean " ^ other_str ^ "?" | None -> "" in
+      typ_error (id_loc id) ("No function type found for " ^ string_of_id id ^ hint_msg)
 
 let get_val_specs env = filter_items env env.global.val_specs
 

src/lib/util.ml

@@ -442,6 +442,27 @@ let list_init len f =
   let rec list_init' len f acc = if acc >= len then [] else f acc :: list_init' len f (acc + 1) in
   list_init' len f 0
 
+let levenshtein_distance ?(osa = false) str1 str2 =
+  let dist = Array.make_matrix (String.length str1 + 1) (String.length str2 + 1) 0 in
+
+  for i = 1 to String.length str1 do
+    dist.(i).(0) <- i
+  done;
+  for j = 1 to String.length str2 do
+    dist.(0).(j) <- j
+  done;
+
+  for i = 1 to String.length str1 do
+    for j = 1 to String.length str2 do
+      let subst_cost = if str1.[i - 1] = str2.[j - 1] then 0 else 1 in
+      dist.(i).(j) <- min (min (dist.(i - 1).(j) + 1) (dist.(i).(j - 1) + 1)) (dist.(i - 1).(j - 1) + subst_cost);
+      if osa && i > 1 && j > 1 && str1.[i - 1] = str2.[j - 2] && str1.[i - 2] = str2.[j - 1] then
+        dist.(i).(j) <- min dist.(i).(j) (dist.(i - 2).(j - 2) + 1)
+    done
+  done;
+
+  dist.(String.length str1).(String.length str2)
+
 let termcode n = if !opt_colors then "\x1B[" ^ string_of_int n ^ "m" else ""
 
 let bold str = termcode 1 ^ str

src/lib/util.mli

@@ -219,6 +219,12 @@ val fold_left_index_last : (int -> bool -> 'a -> 'b -> 'a) -> 'a -> 'b list -> '
 
 val list_init : int -> (int -> 'a) -> 'a list
 
+(** Compute the levenshtein distance between two strings using the
+    Wagner–Fischer algorithm. If [~osa] is true computes the optimal
+    string alignment distance, which is similar but allows swaps as a
+    single action. *)
+val levenshtein_distance : ?osa:bool -> string -> string -> int
+
 (** {2 Files} *)
 
 (** [copy_file src dst] copies file [src] to file [dst]. Only files are supported,

test/typecheck/fail/no_function.expect

@@ -0,0 +1,7 @@
+Type error:
+fail/no_function.sail:10.10-17:
+10 |  let _ = foo_baz()
+   |          ^-----^
+   | No function type found for foo_baz
+   | 
+   | Did you mean foo_bar?

test/typecheck/fail/no_function.sail

@@ -0,0 +1,11 @@
+default Order dec
+
+$include <prelude.sail>
+
+val foo_quux : unit -> unit
+
+val foo_bar : unit -> unit
+
+function test() -> unit = {
+  let _ = foo_baz()
+}

test/typecheck/fail/no_function2.expect

@@ -0,0 +1,7 @@
+Type error:
+fail/no_function2.sail:8.10-17:
+8 |  let _ = foo_baz()
+  |          ^-----^
+  | No function type found for foo_baz
+  | 
+  | Did you mean foo_quux?

test/typecheck/fail/no_function2.sail

@@ -0,0 +1,9 @@
+default Order dec
+
+$include <prelude.sail>
+
+val foo_quux : unit -> unit
+
+function test() -> unit = {
+  let _ = foo_baz()
+}

test/typecheck/fail/no_function3.expect

@@ -0,0 +1,5 @@
+Type error:
+fail/no_function3.sail:6.10-17:
+6 |  let _ = foo_baz()
+  |          ^-----^
+  | No function type found for foo_baz

test/typecheck/fail/no_function3.sail

@@ -0,0 +1,7 @@
+default Order dec
+
+$include <prelude.sail>
+
+function test() -> unit = {
+  let _ = foo_baz()
+}