class IsComplex a where
isComplex :: a -> Bool
data Simple = Myint Int | Mydouble Double | Mystring String deriving (Eq, Show)
data Complex = Struct String [(String,Simple)] deriving (Eq, Show)
instance IsComplex Simple where
isComplex _ = False
instance IsComplex Complex where
isComplex _ = True
is_complex :: IsComplex a => a -> Bool
is_complex a = isComplex a
Refactorings
No refactoring yet !
headcrab.myopenid.com
December 8, 2009, December 08, 2009 22:19, permalink
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Have written this variation. Usage:
*Main> is_complex (UserSimple $ Myint 1)
False
*Main> is_complex (UserComplex $ Struct "one" [("two", Myint 3)])
True
data Simple = Myint Int | Mydouble Double | Mystring String deriving (Eq, Show) data Complex = Struct String [(String,Simple)] deriving (Eq, Show) data UserType = UserSimple Simple | UserComplex Complex is_complex :: UserType -> Bool is_complex (UserSimple a) = False is_complex (UserComplex a) = True
liamoc
December 26, 2009, December 26, 2009 14:47, permalink
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-- Your second example is completely contrived, seeing as the user has to annotate a type anyway, you could just write: is_complex_simple = False is_complex_complex = True -- As for answering your question, there is in fact no easy way to select based on types other than type-class predicate dictionaries (as you have done) as all other type information vanishes at compile time. This isn't really a common usage though for type-classes.
For two given data types, Simple and Complex, define a function which returns whether the variable is of type Complex.
A student asked this on a forum some time ago, and the only solution I could come up with is this. It works, but I wonder: is there another way to do it? is there a _better_ way to do it? is there a possible practical significance or is the task totally contrived?
Example of usage (in GHCi):
*Main> is_complex (Struct "one" [("two", Mystring "three"), ("four", Mystring "five")])
True
*Main> is_complex (Myint 3)
False