sometimes use List.*_opt instead of functions raising Not_found

.github/workflows/test.yml

@@ -17,8 +17,6 @@ jobs:
         - 4.07.1
         - 4.06.1
         - 4.05.0
-        - 4.04.2
-        - 4.03.0
 
     steps:
     - uses: actions/checkout@v2

markup.opam

@@ -13,7 +13,7 @@ dev-repo: "git+https://github.com/aantron/markup.ml.git"
 
 depends: [
   "dune" {>= "2.7.0"}
-  "ocaml" {>= "4.03.0"}
+  "ocaml" {>= "4.05.0"}
   "uchar"
   "uutf" {>= "1.0.0"}
 

src/namespace.ml

@@ -21,10 +21,9 @@ struct
   type context = context_entry ref
 
   let parse qualified_name =
-    try
-      let colon_index = String.index qualified_name ':' in
-      if colon_index = 0 then
-        raise Not_found;
+    match String.index_opt qualified_name ':' with
+    | None | Some 0 -> ("", qualified_name)
+    | Some colon_index ->
       let prefix = String.sub qualified_name 0 colon_index in
       let suffix =
         String.sub qualified_name
@@ -33,8 +32,6 @@ struct
       in
       prefix, suffix
 
-    with Not_found -> ("", qualified_name)
-
   let init top_level =
     let f = function
       | "xml" -> Some xml_ns
@@ -127,15 +124,13 @@ struct
     in
 
     let prefix =
-      try
-        Some (candidate_prefixes |> List.find (fun prefix ->
-          (allow_default || prefix <> "") &&
-           begin
-            try StringMap.find prefix !(fst context).prefix_to_namespace =
-              namespace
-            with Not_found -> false
-           end))
-      with Not_found -> None
+      candidate_prefixes |> List.find_opt (fun prefix ->
+        (allow_default || prefix <> "") &&
+         begin
+          try StringMap.find prefix !(fst context).prefix_to_namespace =
+            namespace
+          with Not_found -> false
+         end)
     in
 
     let prefix =