Question

I am looking at this example using getOpts, and one portion of it really baffles me: the syntax of field labels.

First, this seems simple enough, creating a data type and declaring the initial values:

data Options = Options  { optVerbose    :: Bool
                        , optInput      :: IO String
                        , optOutput     :: String -> IO ()
                        }

startOptions :: Options
startOptions =  Options { optVerbose    = False
                        , optInput      = getContents
                        , optOutput     = putStr
                        }

Then getOpt is used to go through the options and determine the actual parameters for the running program using a foldl command… and then this let expression frustrates me:

let Options { optVerbose = verbose
            , optInput = input
            , optOutput = output   } = opts

The boolean and functions verbose, input, and output are then used after this. In most of the programming languages I’m more familiar with, this step would be written something like so:

verbose = opts.optVerbose
input   = opts.optInput
output  = opts.optOutput

Is Haskell’s behavior here documented someplace?

Answer 1

It’s just normal pattern-matching — just like when you write let (x:xs) = someList and it assigns the first element to x and the rest of the list to xs.

If you wanted, you could write:

let verbose = optVerbose opts
    input   = optInput opts
    output  = optOutput opts

Pattern-matching is everywhere in Haskell and the ML family, but not as common in other languages.