Question

I have a snippet of code that looks something like this:

data SomeData = A | B | C | D | E deriving (Show, Enum)

and I want to be able to map certain values to these types easily. Say I wanted to have chars mapped to a SomeData type, I might do it like so:

mappings = zip "abcde" [A, B, C, D, E]

This would produce [('a',A),('b',B)...] and so on. I was hoping that the Enum type class might allow me to be able to do this easily by converting the data type into the list I described above ([A, B, C, D, E]) by use of some convenience function.

Is there such a function in the standard library? Or would I need to define it myself if I wanted to use something like this?

Answer 1

Generally, I’d do

mappings = zip "abcde" [A..]

but I’d be very tempted to go with

mappings = zip ['a' ..] [A ..]

so that if I added other letters later, the code would update.

Going with maximally future-proof work I’d define

import Data.Char

data SomeData = A | B | C | D | E deriving (Show, Enum, Bounded)

allData :: SomeData
allData = enumFrom minBound

mapppings = zip (map (toLower.head.show) allData) allData

where the only assumtion I’m making about the future is that it consists of single letters.

However, this is playing and not in fact all that clear.