What is the showS trick in Haskell?

Question

I have seen references to the showS trick to build strings (e.g., in this discussion), but I have never seen a good description of it.

What is the showS trick?

Answer 1

In the standard library, ShowS is defined as:

type ShowS = String -> String 

This is a difference list. The trick is that a string xs is represented as a ShowS by the function that prepends it to any other list: (xs ++). This allows efficient concatenation, avoiding the problems of nested left-associative concatenation (i.e. ((as ++ bs) ++ cs) ++ ds). For example:

hello = ("hello" ++) world = ("world" ++)  -- We can "concatenate" ShowS values simply by composing them: helloworld = hello . world  -- and turn them into Strings by passing them an empty list: helloworld' = helloworld "" 

It's called ShowS because it's used in the implementation of the standard Show typeclass to allow efficient showing of large, deeply-nested structures; as well as show, you can implement showsPrec, which has the type:

showsPrec :: (Show a) => Int -> a -> ShowS 

This allows handling of operator precedence, and returns a ShowS value. The standard instances implement this instead of show for efficiency; show a is then defined in terms of it, as showsPrec 0 a "". (This default definition is in the Show typeclass itself, so you can just implement showsPrec for a complete instance.)

Answer 2

showS uses the difference list approach to efficiently concatenate individual components of the shown value. The function takes the value to be shown, and a string to append to the result. The appended string is passed all the way down to the right-most sub-value until it reaches a leaf, where it is actually appended.

There's a description of difference lists (including showS) here http://www.haskell.org/haskellwiki/Difference_list