How do I parse a String to a Float?

Question

I need to consume a json source that represents floats as strings* and I can't figure out how.

It is almost easy:

Json.Decode.map String.toFloat Json.Decode.string

However, that produces a Maybe Float and I'd prefer it fail altogether if it cant decode the string.

(*) The reason for this is that the real datatype is Decimal, so "1.5" != "1.50". My application doesn't have to care though.

Answer 1

You can either install elm-community/json-extra and use Json.Decode.Extra.parseFloat

or just copy its implementation

fromMaybe : String -> Maybe a -> Decode.Decoder a
fromMaybe error val =
    case val of
        Just v ->
            Decode.succeed v

        Nothing ->
            Decode.fail error

parseFloat : Decode.Decoder Float
parseFloat =
    Decode.string |> Decode.andThen (String.toFloat >> fromMaybe "failed to parse as float")