Add oneOf.

src/QueryParams.elm

@@ -1,4 +1,4 @@
-module QueryParams exposing (Parser, QueryParams, fromString, optionalString, parse, string, strings, toDict)
+module QueryParams exposing (Parser, QueryParams, fail, fromString, map2, oneOf, optionalString, parse, string, strings, succeed, toDict)
 
 import Dict exposing (Dict)
 import Url
@@ -12,6 +12,44 @@ type Parser a
     = Parser (Dict String (List String) -> Result String a)
 
 
+succeed : a -> Parser a
+succeed value =
+    Parser (\_ -> Ok value)
+
+
+fail : String -> Parser a
+fail errorMessage =
+    Parser (\_ -> Err errorMessage)
+
+
+oneOf : List (Parser a) -> Parser a
+oneOf parsers =
+    Parser
+        (tryParser parsers)
+
+
+tryParser : List (Parser a) -> Dict String (List String) -> Result String a
+tryParser parsers dict =
+    case parsers of
+        [] ->
+            Err ""
+
+        (Parser nextParser) :: otherParsers ->
+            case nextParser dict of
+                Ok okValue ->
+                    Ok okValue
+
+                Err _ ->
+                    tryParser otherParsers dict
+
+
+map2 : (a -> b -> combined) -> Parser a -> Parser b -> Parser combined
+map2 func (Parser a) (Parser b) =
+    Parser <|
+        \dict ->
+            Result.map2 func (a dict) (b dict)
+
+
 optionalString : String -> Parser (Maybe String)
 optionalString key =
     custom key

tests/QueryParamsTests.elm

@@ -42,4 +42,24 @@ all =
                             [ ( "q", [ "mySearch1", "mySearch2" ] )
                             ]
                         )
+        , test "oneOf with no parsers" <|
+            \() ->
+                "q=mySearch1&q=mySearch2"
+                    |> QueryParams.fromString
+                    |> QueryParams.parse (QueryParams.oneOf [])
+                    |> Expect.equal (Err "")
+        , test "oneOf with two parsers" <|
+            \() ->
+                "first=Jane&last=Doe"
+                    |> QueryParams.fromString
+                    |> QueryParams.parse
+                        (QueryParams.oneOf
+                            [ QueryParams.string "fullName"
+                            , QueryParams.map2
+                                (\first last -> first ++ " " ++ last)
+                                (QueryParams.string "first")
+                                (QueryParams.string "last")
+                            ]
+                        )
+                    |> Expect.equal (Ok "Jane Doe")
         ]