You have to capitalize your type names and data constructors:
data MyTuple a b c d = My4Tuple a b c d
| My3Tuple a b c
| My2Tuple a b
| My1Tuple a
deriving (Show)
On Wed, Jan 14, 2009 at 11:44 AM, David Schonberger
Hi all. I'm attempting exercise 4.6 from Daume's Yet Another Haskell Tutorial:
Exercise 4.6
Write a datatype Tuple which can hold one, two, three or four elements,
depending on the constructor (that is, there should be four constructors, one for each
number of arguments). Also provide functions
tuple1 through tuple4 which take a
tuple and return
Just the value in that position, or Nothing if the number is invalid (i.e., you ask for the tuple4 on a tuple holding only two elements)
For starters, I began with the following, stored in a file called MyTuple.hs:
data myTuple a b c d = my4Tuple a b c d | my3Tuple a b c | my2Tuple a b | my1Tuple a deriving (Show)
When I load this into WinHugs I get the following error:
file:.\MyTuple.hs:1 - Syntax error in data declaration (unexpected symbol "a")
However I get the same error if I reduce it to a single data constructor, in an effort to create just a quadruple:
data myTuple a b c d = my4Tuple a b c d
What is even more odd is that I have the following code in a file called Pair.hs:
{-data Pair a b = Pair a b deriving (Show) pairFst (Pair x y) = x pairSnd (Pair x y) = y data Quadruple a b c d = Quadruple a a b b deriving (Show) fstPairofQuad (Quadruple a b c d) = [a,b] sndPairofQuad (Quadruple a b c d) = [c,d] -}
data Quad a b c d = QuadFour a b c d | QuadThree a b c | QuadTwo a b | QuadOne a deriving (Show)
{- fstPairofQuad (QuadFour a b c d) = (a,b) mdlPairofQuad (QuadFour a b c d) = (b,c) sndPairofQuad (QuadFour a b c d) = (c,d) fstLstofQuad (QuadFour a b c d) = (a,d)-}
Whether I just keep Quad visible in the file or I take away both pairs of comment braces, when I load that file into WinHugs it works fine, and I have no trouble constructing a Quad or using any of the QuadFour utility fcns (which, admittedly are not what is called for in the exercise... but that's probably besides the point).
What in the world might be causing the error in the MyTuple file?
Thanks, David
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