Stupid Haskell Tricks

Let’s say that you really, really want some notion of objected oriented programming. So let’s make a class that represents a name, and some simple method calls on it:

data S = S { name :: String } deriving (Show) firstname s = (words (name s))!!0 lastname s = (words (name s))!!1

But, dammit, you want to invoke it like you would in C++. So define a function:

*Main> let (-->) x f = f x *Main> let test = S "George Washington" *Main> test --> firstname "George" *Main> test --> lastname "Washington"

(It’s tempting to use `.`, but it conflicts with the Prelude. Also note that you could define it as “(–>) = flip (\$)“). But what if it takes more than one parameter?

*Main> let flip_concat t s = intercalate t $ reverse.words $ name s *Main> test --> flip_concat ", " "Washington, George" *Main> test --> flip_concat(", ") "Washington, George"

… although you might not like that anything can be applied to (–>).

*Main> [1,2,3] --> length 3 class Deref a where (-->) :: a -> (a -> b) -> b x --> f = f x instance Deref S *Main> [1,2,3] --> length :1:0: No instance for (Deref [t]) arising from a use of `-->' at :1:0-17 Possible fix: add an instance declaration for (Deref [t]) In the expression: [1, 2, 3] --> length In the definition of `it': it = [1, 2, 3] --> length

You can even use tuple passing to make it look even more like a typical call.

*Main> let pretty (pre, mid, post) s = pre ++ (firstname s) ++ mid ++ (lastname s) ++ post *Main> test --> pretty ("<", ", ", ">") ""

… although that makes it harder to curry.

Discussion

Comments are moderated whenever I remember that I have a blog.