Add function to decode the contetns of a JSON string with another decoder

src/Json/Decode.elm

@@ -1,5 +1,5 @@
 module Json.Decode exposing
-  ( Decoder, string, bool, int, float
+  ( Decoder, string, stringAs, bool, int, float
   , nullable, list, array, dict, keyValuePairs, oneOrMore
   , field, at, index
   , maybe, oneOf
@@ -23,7 +23,7 @@ JSON decoders][guide] to get a feel for how this library works!
 @docs field, at, index
 
 # Inconsistent Structure
-@docs maybe, oneOf
+@docs maybe, oneOf, stringAs
 
 # Run Decoders
 @docs decodeString, decodeValue, Value, Error, errorToString
@@ -75,6 +75,53 @@ string =
   Elm.Kernel.Json.decodeString
 
 
+{-| Decode the contents a JSON string with another decoder. This can be useful
+if the JSON contains values that are really numbers, but exists as strings. For
+example, say you want to decode the list stringified numbers.
+
+    sneakyFloat : Decoder Float
+    sneakyFloat =
+      stringAs float
+
+    -- decodeString (list sneakyFloat) "[\"1.5\", \"42.0\"]" == Ok [1.5, 42.0]
+
+Unfortunately, there could be times like this when you have to work with JSON
+that uses strings to represent floats instead of just regular JSON numbers. If
+you will need to do actual arithmetic on them, like multiplying them together,
+then this functions allows you to parse them correctly with ease.
+
+It can be used together with `oneOf` to handle cases when a number is sometimes
+represented by a JSON string and sometimes an actual JSON number.
+
+NOTE: When the these values are integers as strings, then there is a valid
+reason NOT to decode them into integers. JavaScript uses floating point types
+to represent integers as well, so there is no guarantee that it will be exact.
+For example for unique IDs, it is better to use strings in JavaScript.
+-}
+stringAs : Decoder value -> Decoder value
+stringAs decoder =
+  string
+    |> andThen
+      (\str ->
+        case decodeString decoder str of
+          Err err ->
+            failWithError err
+
+          Ok val ->
+            succeed val
+      )
+
+
+{-| TODO: write a proper version of this function.
+Depends on [#25][issue-25]
+
+[issue-25]: https://github.com/elm/json/issues/25
+-}
+failWithError : Error -> Decoder a
+failWithError =
+  fail << errorToString
+
+
 {-| Decode a JSON boolean into an Elm `Bool`.
 
     decodeString bool "true"              == Ok True

tests/Json.elm

@@ -12,6 +12,7 @@ tests : Test
 tests =
     describe "Json decode"
         [ intTests
+        , stringAsTests
         , customTests
         ]
 
@@ -54,6 +55,30 @@ intTests =
             ]
 
 
+stringAsTests : Test
+stringAsTests =
+    let
+        testStringAs decoder val str =
+            case Json.decodeString (Json.stringAs decoder) str of
+                Ok _ ->
+                    Expect.equal val True
+
+                Err _ ->
+                    Expect.equal val False
+        int = Json.int
+        float = Json.float
+    in
+        describe "Jsong decode stringAs"
+            [ test "int string -> int" <| testStringAs int True "\"3\""
+            , test "float string -> int" <| testStringAs int False "\"3.0\""
+            , test "int -> int" <| testStringAs int False "3"
+            , test "float -> int" <| testStringAs int False "3.0"
+            , test "int string -> float" <| testStringAs float False "\"3\""
+            , test "float string -> float" <| testStringAs float True "\"3.0\""
+            , test "int -> float" <| testStringAs float False "3"
+            , test "float -> float" <| testStringAs float False "3.0"
+            ]
+
 customTests : Test
 customTests =
     let