How do I improve QuickCheck and Parsec debugging?

Question

I am using Haskell and Parsec to parse a file format. My parsing function looks something like:

parseInput :: String -> Model
parseInput input = ...

data Model = Model { mNumV :: Int, mNumF :: Int, ... }

In order to test this, I am using QuickCheck. I have defined an Arbitrary instance that generates a String representing the contents of a formatted file:

instance Arbitrary File where
    arbitrary = ...

data File = File { fContents :: String, fNumV :: Int, fNumF :: Int, ... }

One of my properties might check to determine if mNumV == fNumV after parsing the arbitrary String. This works well - when it works.

But if something fails, Parsec throws an error similar to:

*** Failed (after 1 test):
Exception:
  (line 302, column 3):
  unexpected "\n"
  expecting space

This is useful - however, after the test fails the contents of the arbitrary file disappear. I can't go in and reference line 302.

The only alternative that I can see is to print the fContents of each arbitrary file after each test - but that seems like a terrible idea. The same goes for routing every arbitrary file to a file on disk for later reference.

Is there a common way around this?

Answer 1

You can use whenFail to print the offending string (or dump it to a file) upon failure.