It happens very often that I want to convert a number of values to strings and concatenate those strings into one. No surprise there, of course. Well, I'd prefer to do it efficiently and with as little code as necessary.
{-# LANGUAGE TypeSynonymInstances #-} module ShowsDemo where
Let's say I want to generate the string "(42 abc)" starting with a number and a string stored in variables.
n = 42 :: Int s = "abc"
What are my options? There's the obvious approach that's described in every tutorial, book, and research paper (for didactic purposes, of course).
ex1 = "(" ++ show n ++ " " ++ s ++ ")"
It's pretty concise, but it's horribly inefficient due to the use of (++). Then, there's the ShowS approach.
ex2 = showChar '(' . shows n . showChar ' ' . showString s . showChar ')' $ ""
This is more efficient, but now the code has bloated up a lot. Why can't I have my cake and eat it, too? I want to write with as little code as |ex1| (or less if possible), and I want it to be as efficient as |ex2|. I propose this example as an improvement.
ex3 = '(' .+. n .+. ' ' .+. s .$. ')'
It uses a class I'm calling |Shows|. The class has one method that simply converts a value to the type |ShowS|, where |ShowS| is a type synonym for |String -> String| and is defined in the Prelude.
class Shows a where toShows :: a -> ShowS
Notice the lack of context involving the |Show| class. That's important, because it allows us to create more instances than we could if we were restricted by |(Show a) => ...|, esp. the |ShowS| instance below. The instances for types are all very simple. Most will appear like the instance for |Int|.
instance Shows Int where toShows = shows
Since we don't have |Show| in the class context above, we can't make this a default method. We need a few special instances for |Char| and |String| to make these types convenient to use in the expected way.
instance Shows Char where toShows = showChar
instance Shows String where toShows = showString
We also need an instance for |ShowS| in order to facilitate concatenation.
instance Shows ShowS where toShows = id
Now, we define a few operators that use |toShows| and make our lives easier and our code more concise. The |(.+.)| replaces list appending, |(++)|, in |ex1| and function composition, |.|, in |ex2|.
infixl 5 .+. (.+.) :: (Shows a, Shows b) => a -> b -> ShowS a .+. b = toShows a . toShows b
The |(.$.)| replaces the need for |($)| in |ex2|.
infixl 4 .$. (.$.) :: (Shows a, Shows b) => a -> b -> String a .$. b = (a .+. b) ""
I would find something like this very useful. I'm guessing the idea can be applied to |ByteString| as well. Does it exist in some other form? Sean