Haskell中不带参数的匿名函数

Question

Could I define an anonymous function without parameter in Haskell?

I have a block of code repeated in several branches. Those code refer to several values outside.

Purpose 0: Define a function do the job in the codeBlock.

Purpose 1: Don't repeat codeBlock twice.

Purpose 2: Don't pass d1..d4 to the function. Avoid passing file and time even better.

f event d1 d2 d3 d4 =
  case event of
    (Modified file time) -> do
       codeBlock file time d1 d2 d3 d4 
    (Added file time) -> do
       codeBlock file time d1 d2 d3 d44
    _ -> return ()

Answer 1

There is no such thing as a function without parameters (anonymous or otherwise). That's just a value (and in Haskell, actions like main :: IO () are also just values).

For sure, a value can be defined anywhere (like a function); however if you want to reuse it in more than one place you should not make it anonymous but give it a (locally scoped) name:

f event d1 d2 d3 d4 =
  case event of
    (Modified file time) -> do
       defaultAction time
    (Added file time) -> do
       defaultAction time
    _ -> return ()
 where defaultAction time = do
           codeBlock file time d1 d2 d3 d4

BTW, do blocks with only a single statement are equivalent to just that statement alone, ie you could also write

f event d1 d2 d3 d4 =
  case event of
    (Modified file time) -> defaultAction time
    (Added file time) -> defaultAction time
    _ -> return ()
 where defaultAction time = codeBlock file time d1 d2 d3 d4