Expectation for a decoder result.

• FailsToDecode - expect the decoder to fail, the failure message can be anything
• FailsToDecodeWith String - expect the decoder to fail with a specific message
• DecodesTo a - expect the decoder to succeed, decoding to the provided value

Exercise a decoder over a list of input/expectation pairs.

For example

describeDecoder "int"
  Json.Decode.int
  [ ( "", FailsToDecode )
  , ( "foo", FailsToDecode )
  , ( "1", DecodesTo 1 )
  , ( "1.5", FailsToDecode )
  ]

Exercise a decoder with a JSON encoded string.

For example

testDecoder Json.Decode.string
  "\"foo\""
  (DecodesTo "foo")