Json.Decode.customDecoder swallows Err strings

[noahzgordon]

The function Json.Decode.customDecoder has the following type:

Decoder a -> (a -> Result String b) -> Decoder b

My expectation as a user is that the decoder produced by customDecoder decodes an initial payload using the first argument, then maps the result into the second argument. However, when the second argument is applied and its result resolves to an Err string, the string is discarded. No matter what, when a custom decoder fails, the string value of Err is "Expecting something custom but instead got: ".

You can reproduce this by entering the following into elm repl:

import Json.Decode exposing (customDecoder, decodeString, string, (:=))

jsonString = """{ "foo": "bar" }"""

myDecoder = customDecoder ("foo" := string) (\_ -> Err "I want to see this message!")

decodeString myDecoder jsonString
-- Err "Expecting something custom but instead got: {\"foo\":\"bar\"}"

[evancz]

Should be fixed by in 7f6e518

[evancz]

Should be fixed by in 7f6e518