fix syntax

Signed-off-by: Etienne Millon <me@emillon.org>

src/dune_lang/package_name.ml

@@ -32,35 +32,40 @@ module Strict = struct
     let description_of_valid_string =
       Some
         (Pp.textf
-           "Package names start with a letter and can contain letters, \
-            numbers, '-', '_' and '+'")
-
-    let is_valid_char ~at_start = function
-      | 'a' .. 'z' | 'A' .. 'Z' -> true
-      | '0' .. '9' | '-' | '_' | '+' -> not at_start
-      | _ -> false
+           "Package names can contain letters '-', '_' and '+', and need to contain at least a letter.")
 
     let of_string_opt s =
       let open Option.O in
-      let* empty =
-        String.fold_left s ~init:(Some true) ~f:(fun state c ->
-            let* at_start = state in
-            if is_valid_char ~at_start c then Some false else None)
+      let* has_letter =
+        String.fold_left
+          ~f:(fun acc c ->
+            match (acc, c) with
+            | None, _ -> None
+            | _, ('a' .. 'z' | 'A' .. 'Z') -> Some true
+            | _, ('0' .. '9' | '-' | '_' | '+') -> acc
+            | _ -> None)
+          ~init:(Some false) s
       in
-      Option.some_if (not empty) s
+      Option.some_if has_letter s
 
     let make_valid s =
       let b = Buffer.create 0 in
       let emit c = Buffer.add_char b c in
-      let (_ : bool) =
-        String.fold_left s ~init:true ~f:(fun at_start c ->
-            if is_valid_char ~at_start c then emit c
-            else if not at_start then emit '_';
-            false)
+      let has_letter =
+        String.fold_left s ~init:false ~f:(fun acc c ->
+            match c with
+            | 'a' .. 'z' | 'A' .. 'Z' ->
+              emit c;
+              true
+            | '0' .. '9' | '-' | '_' | '+' ->
+              emit c;
+              acc
+            | _ ->
+              emit '_';
+              acc)
       in
-      match Buffer.contents b with
-      | "" -> "a"
-      | s -> s
+      let s = Buffer.contents b in
+      if has_letter then s else "p" ^ s
 
     let hint_valid = Some make_valid
   end)

src/dune_lang/package_name.mli

@@ -16,8 +16,8 @@ include Dune_sexp.Conv.S with type t := t
 include Stringlike with type t := t
 
 module Strict : sig
-  (** A variant that enforces opam package name constraints:
-      [[a-zA-Z][a-zA-Z0-9_+-]*] *)
+  (** A variant that enforces opam package name constraints: all characters are
+      [[a-zA-Z0-9_+-]] with at least a letter. *)
 
   include Stringlike
 

test/blackbox-tests/test-cases/package-name-strict.t

@@ -33,14 +33,6 @@ Validation:
 Leading invalid characters are removed:
 
   $ test '0test'
-  File "dune-project", line 3, characters 7-12:
-  3 |  (name 0test)
-             ^^^^^
-  Error: "0test" is an invalid package name.
-  Package names start with a letter and can contain letters, numbers, '-', '_'
-  and '+'
-  Hint: test would be a correct package name
-  [1]
 
 When all characters are removed, a valid name is suggested:
 
@@ -53,3 +45,19 @@ When all characters are removed, a valid name is suggested:
   and '+'
   Hint: a would be a correct package name
   [1]
+
+A package name can start with a number:
+
+  $ test 0install
+
+But it needs at least a letter:
+
+  $ test 0-9
+  File "dune-project", line 3, characters 7-10:
+  3 |  (name 0-9)
+             ^^^
+  Error: "0-9" is an invalid package name.
+  Package names start with a letter and can contain letters, numbers, '-', '_'
+  and '+'
+  Hint: -9 would be a correct package name
+  [1]