Haskell quickcheck - how to generate only printable strings

Question

I have a set of simple demo programs which encode/decode strings, and want to generate some quickCheck tests for them, but to limit the tests to printable strings only.

Answer 1

The starting point is definitely a genSafeChar generator, which can have type Gen Char. For example:

genSafeChar :: Gen Char
genSafeChar = elements ['a'..'z']

Then you can build that up into a genSafeString generator, e.g. with listOf:

genSafeString :: Gen String
genSafeString = listOf genSafeChar

At this point you have a couple of reasonable choices. Either make a newtype wrapper for String:

newtype SafeString = SafeString { unwrapSafeString :: String }
    deriving Show

instance Arbitrary SafeString where
    arbitrary = SafeString <$> genSafeString

(in this case you might just inline the definition of genSafeString)

and you can then use it something like this:

testWibble (SafeString str) = str == str

Or, you can use forAll at each point you need a safe string:

testWibble = forAll genSafeString $ \str -> str == str

Answer 2

Currently QuickCheck has a PrintableString type, which also has an instance of arbitrary, which means that you can readily generate arbitrary strings with:

arbitraryPrintableString :: Gen String
arbitraryPrintableString = getPrintableString <$> arbitrary