Computer Science Canada

[Haskell-tut] Follow-up: Combining Functions
Author:  wtd [ Fri Jan 07, 2005 8:18 pm ]
Post subject:  [Haskell-tut] Follow-up: Combining Functions
In my introduction to Haskell I createded a simple function greeting, which takes a name as a string and formulates a greeting.

code:
greeting :: String -> String
greeting name = "Hello, " ++ name ++ "!"


And then, I created a greet function which takes a name and prints the greeting for it.

code:
greet :: String -> IO ()
greet name = putStrLn (greeting name)


The latter function I rewrote as:

code:
greet :: String -> IO ()
greet name = putStrLn $ greeting name


Now, the argument "name" should appear quite redundant in that last example. It is. What if, instead, we could simply combine the two functions, "greeting" and "putStrLn".

Haskell provides an easy mechanism for doing so.

code:
greet :: String -> IO ()
greet = putStrLn . greeting


Now, we can make a similar observation about the greeting function.

code:
greeting :: String -> String
greeting name = "Hello, " ++ name ++ "!"


This is really two functions, since operators are just functions. Given the order of evaluation, this could be rewritten:

code:
greeting :: String -> String
greeting name = "Hello, " ++ (name ++ "!")


Now, since we can partially apply functions - give them one argument, and get back a function which takes another argument and gives us the rest - we can rewrite this as:

code:
greeting :: String -> String
greeting = ("Hello, " ++) . (++ "!")


Just as in the original, the function takes a string, appends "!" to it, then prepends "Hello, " to that.

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