pangram 1.4.1.10: Add test case: determine pangram lazily (#795)

* pangram 1.4.1.10: Add test case: determine pangram lazily

Fail if the input stream was touched beyond the first occurrence of each letter.

Many standard solutions achieve this property without much effort, and some don't.

This test case notifies the student that a more optimal solution is within reach.

* Disable the test by default

While `success-standard` passes this test, and a `String`-based
`fail-too-eager` example fails as expected, `success-text` cannot easily
succeed, even if is changed into using `Data.Text.Lazy`:

One can write a `Data.Text.Lazy as LT`-specific test:

    describe "isPangram" $ do
      it "is lazy" $
        isPangram (LT.fromChunks ["a...z", undefined]) `shouldBe` True

But when the input comes via `fromString :: IsString s => String -> s`,
the implementation has little control of the chunking. A failed attempt
to circumvent this,

    newtype ChunkyLazyText = ChunkyLazyText LT.Text

    instance IsString ChunkyLazyText where
      fromString = ChunkyLazyText . LT.fromChunks . map (T.pack . return)

still causes the `T.pack` invocation on `[undefined]`.

And as @bobdalgleish pointed out in [this StackOverflow question][SO],
I'd actually end up testing `fromString`. Since "laziness" doesn't mean
the same thing for `String` as it does for `Data.Text.Lazy` because
chunking works differently, the reliable alternative is to write tests
for each implementation.

Or, in this case, let string-type specific tests remain commented out.

[SO]: https://stackoverflow.com/questions/53784627/testing-laziness-of-isstring-s-s-bool-function

* Clarify that this test is mainly intended for String-based solutions

exercises/pangram/package.yaml

@@ -1,5 +1,5 @@
 name: pangram
-version: 1.4.1.9
+version: 1.4.1.10
 
 dependencies:
   - base

exercises/pangram/test/Tests.hs

@@ -62,4 +62,11 @@ cases = [ Case { description = "sentence empty"
                , input       = "the quick brown fox jumps over with lazy FX"
                , expected    = False
                }
+        {-
+        -- The following test can be enabled for String-based solutions:
+        , Case { description = "determine pangram by terminating as soon as all letters have occurred"
+               , input       = "abcdefghijklmnopqrstuvwxyz" ++ [undefined]
+               , expected    = True
+               }
+        -- -}
         ]