This is a story about a horrific blunder. Thankfully, no Bothans died bringing this information to you.
As I mentioned previously, I wrote a JSON parser to demonstrate how to write a real, live working Haskell program. I started by working off of the pseudo-BNF found on the JSON homepage. From the perspective of the JSON grammar, the constructs it deals with are objects (otherwise known as Maps, hashes, dicts or associative arrays), arrays, strings, numbers, and the three magic values true, false and null.
My first task was to create a data type that captures the values that can be expressed in this language:
With the datatype in place, I then started writing parsing functions to build objects, arrays, and so on. Pretty soon, I had a JSON parser that passed the validation tests.data Value = String String
| Number Double
| Object (Map String Value)
| Array [Value]
I used this piece of working Haskell code during my presentation, highlighting how all the parts worked together -- the parsers that returned specific kinds of Value types, those that returned String values, and so on.
Pretty soon I got tongue tied, talking about how Value was a type, and why String was a type in some contexts, and a data constructor for Value types in other contexts. And how Number wasn't a number, but a Value.
I'm surprised anyone managed to follow that code.
The problem, as I see it, is that I was so totally focused on the JSON domain that I didn't think about the Haskell domain. My type was called Value, because that's what it's called in the JSON grammar. It never occurred to me as I was writing the code that a type called Value is pretty silly. And, because types and functions are in separate namespaces, I never noticed that the data constructor for strings was called String.
Thankfully, the code was in my editor, so I changed things on the fly during the presentation to make these declarations more (ahem) sane:
I think that helped to clarify that String is a pre-defined type, and JsonString is a value constructor that returns something of type JsonValue.data JsonValue = JsonString String
| JsonNumber Double
| JsonObject (Map String JsonValue)
| JsonArray [JsonValue]
deriving (Show, Eq)
When I gave this presentation again a couple of weeks ago, the discussion around this JSON parser was much less confusing.
Lesson learned: let the compiler and another person read your code to check that it makes sense. ;-)