Better error messages for incomplete probability calls

src/frontend/Semantic_check.ml

@@ -290,9 +290,30 @@ let semantic_check_fn_normal ~is_cond_dist ~loc id es =
     | Some _ ->
         (* Check that Funaps are actually functions *)
         Semantic_error.returning_fn_expected_nonfn_found loc id.name |> error
-    | None ->
-        Semantic_error.returning_fn_expected_undeclaredident_found loc id.name
-        |> error)
+    | None -> (
+      match List.rev (String.split id.name ~on:'_') with
+      | (("lupdf" | "cdf" | "lcdf" | "lccdf" | "rng") as suffix) :: tl ->
+          let prefix = String.concat ~sep:"_" (List.rev tl) in
+          let known_families =
+            List.map
+              ~f:(fun (_, y, _, _) -> y)
+              Stan_math_signatures.distributions
+          in
+          let is_known_family s =
+            List.mem known_families s ~equal:String.equal
+          in
+          if is_known_family prefix then
+            Semantic_error.returning_fn_expected_undeclared_dist_suffix_found
+              loc (prefix, suffix)
+            |> error
+          else
+            Semantic_error.returning_fn_expected_undeclaredident_found loc
+              id.name
+            |> error
+      | _ ->
+          Semantic_error.returning_fn_expected_undeclaredident_found loc
+            id.name
+          |> error ))
 
 (* Stan-Math function application *)
 let semantic_check_fn_stan_math ~is_cond_dist ~loc id es =

src/middle/Semantic_error.ml

@@ -33,6 +33,7 @@ module TypeError = struct
     | ReturningFnExpectedNonReturningFound of string
     | ReturningFnExpectedNonFnFound of string
     | ReturningFnExpectedUndeclaredIdentFound of string
+    | ReturningFnExpectedUndeclaredDistSuffixFound of string * string
     | NonReturningFnExpectedReturningFound of string
     | NonReturningFnExpectedNonFnFound of string
     | NonReturningFnExpectedUndeclaredIdentFound of string
@@ -168,6 +169,13 @@ module TypeError = struct
           "A returning function was expected but an undeclared identifier \
            '%s' was supplied."
           fn_name
+    | ReturningFnExpectedUndeclaredDistSuffixFound (prefix, suffix) ->
+        Fmt.pf ppf
+          "A function was expected but an unknown identifier %s was recieved. \
+           This appears to be part of the %s family of distributions, for \
+           which the %s suffix in not implemented."
+          (String.concat ~sep:"_" [prefix; suffix])
+          prefix suffix
     | NonReturningFnExpectedUndeclaredIdentFound fn_name ->
         Fmt.pf ppf
           "A non-returning function was expected but an undeclared identifier \
@@ -496,6 +504,12 @@ let returning_fn_expected_nonfn_found loc name =
 let returning_fn_expected_undeclaredident_found loc name =
   TypeError (loc, TypeError.ReturningFnExpectedUndeclaredIdentFound name)
 
+let returning_fn_expected_undeclared_dist_suffix_found loc (prefix, suffix) =
+  TypeError
+    ( loc
+    , TypeError.ReturningFnExpectedUndeclaredDistSuffixFound (prefix, suffix)
+    )
+
 let nonreturning_fn_expected_returning_found loc name =
   TypeError (loc, TypeError.NonReturningFnExpectedReturningFound name)
 

src/middle/Semantic_error.mli

@@ -34,6 +34,9 @@ val returning_fn_expected_nonfn_found : Location_span.t -> string -> t
 val returning_fn_expected_undeclaredident_found :
   Location_span.t -> string -> t
 
+val returning_fn_expected_undeclared_dist_suffix_found :
+  Location_span.t -> string * string -> t
+
 val illtyped_reduce_sum :
      Location_span.t
   -> string