resistor-color-duo: Change to value :: (Color, Color) -> Int

TL;DR:
 - I don't think we should be teaching people to write functions like this.
 - The less controversial action would be to reject this PR in favor of one
   where either

        value :: [Color] -> Maybe Int
        value :: [Color] -> Either String Int
        value :: [Color] -> Either ColorError Int

Quick chronological recap of PRs that provide necessary context:

 - 2019-03-08: exercise created in exercism/problem-specifications#1466
 - 2019-03-12: exercise ported to Haskell track in #808
 - 2019-08-17: third-band test case and formulation "brown-green-violet should
   return 15 too, ignoring the third color." + one other similar formulation
   added to README.md in exercism/problem-specifications#1569.
 - 2019-10-01: third-band test case added to Haskell track in #849.

When this exercise was ported to Haskell track, it was decided to not address
error handling in this exercise via `Maybe Int` or similar, which otherwise
would be necessary since `value` is not defined for `[Color]` of length 0 or 1.

The alternative proposed here, to limit the input to exactly two bands, does
not appear to be mentioned.

In the meantime, the third-band test case (and formulations) have made `value`
explicitly more ambiguous: Unlike 0 or 1 bands, which is undefined, the third
band should be ignored, and >3 bands are undefined. Presumably they should be
ignored as well, but following this logic, should 0 or 1 bands also be ignored?
Why, exactly, are we creating a function so vulnerable to wrong input and not
let it handle validation gracefully?

README.md explicitly asks to disregard a third color in two places. Given this
proposal, those messages become meaningless; you can only specify two bands.

I can think of these ways to address the deficiency that this change causes:

 - Remove the lines from README.md, breaking a track invariant and CI check.
 - Apply a track-specific hint saying how this exercise is adapted to Haskell
   track in this way.
 - Don't change the description; confuse students. ("What third band?!")
 - Reject this PR in favor of another solution, as highlighted above.
 - Something else entirely.

The Read instance is removed from the stub as it isn't necessary.

This changes the version from 2.1.0.2 to 0.1.0.3, because the three-band test
case is removed, which makes the test suite no longer reflect canonical tests.

exercises/resistor-color-duo/examples/success-standard/src/ResistorColors.hs

@@ -11,19 +11,19 @@ data Color =
   | Violet
   | Grey
   | White
-  deriving (Eq, Show, Read)
+  deriving (Eq, Show)
 
-convert :: Color -> Int
-convert Black = 0
-convert Brown = 1
-convert Red = 2
-convert Orange = 3
-convert Yellow = 4
-convert Green = 5
-convert Blue = 6
-convert Violet = 7
-convert Grey = 8
-convert White = 9
+value1 :: Color -> Int
+value1 Black = 0
+value1 Brown = 1
+value1 Red = 2
+value1 Orange = 3
+value1 Yellow = 4
+value1 Green = 5
+value1 Blue = 6
+value1 Violet = 7
+value1 Grey = 8
+value1 White = 9
 
-value :: [Color] -> Int
-value = read . concatMap (show . convert) . take 2
+value :: (Color, Color) -> Int
+value (a, b) = 10 * value1 a + value1 b

exercises/resistor-color-duo/package.yaml

@@ -1,5 +1,5 @@
 name: resistor-color-duo
-version: 2.1.0.2
+version: 0.1.0.3
 
 dependencies:
   - base

exercises/resistor-color-duo/src/ResistorColors.hs

@@ -11,7 +11,7 @@ data Color =
   | Violet
   | Grey
   | White
-  deriving (Eq, Show, Read)
+  deriving (Eq, Show)
 
-value :: [Color] -> Int
-value cs = error "You need to implement this function."
+value :: (Color, Color) -> Int
+value (a, b) = error "You need to implement this function."

exercises/resistor-color-duo/test/Tests.hs

@@ -20,29 +20,25 @@ specs = describe "value" $ for_ cases test
         assertion   = value input `shouldBe` expected
 
 data Case = Case { description :: String
-                 , input       :: [Color]
+                 , input       :: (Color, Color)
                  , expected    :: Int
                  }
 
 cases :: [Case]
 cases = [ Case { description = "Brown and black"
-               , input       = [Brown, Black]
+               , input       = (Brown, Black)
                , expected    = 10
                }
         , Case { description = "Blue and grey"
-               , input       = [Blue, Grey]
+               , input       = (Blue, Grey)
                , expected    = 68
                }
         , Case { description = "Yellow and violet"
-               , input       = [Yellow, Violet]
+               , input       = (Yellow, Violet)
                , expected    = 47
                }
         , Case { description = "Orange and orange"
-               , input       = [Orange, Orange]
+               , input       = (Orange, Orange)
                , expected    = 33
                }
-        , Case { description = "Ignore additional colors"
-               , input       = [Green, Brown, Orange]
-               , expected    = 51
-               }
         ]