Function with type synonym

Question

Sorry for asking as potentially silly question, but returning to Haskell to do some conversion from one database package to a different one, I find myself a bit puzzled about how to do this properly.

In the Database.SQLite3 module, there is an execWithCallback with type

execWithCallback :: Database -> Text -> ExecCallback -> IO ()

Now, the callback is defined as

type ExecCallback = ColumnCount -> [Text]-> [Maybe Text] -> IO ()

that is, a function with type ExecCallback My silly test code compiles and runs correctly:

{-# LANGUAGE OverloadedStrings #-}

import Database.SQLite3
import Data.Text

cb :: ColumnCount -> [Text] -> [Maybe Text] -> IO ()
cb  n cnl ct = do print $ cnl !! 1
                  return ()

main = do
  dh <- open "fileinfo.sqlite"
  execWithCallback dh "select * from files;" cb
  close dh

but then, what is the point of the type??? And, how do I specify that cb is an ExecCallback??

Answer 1

In Haskell, with type you define a type synonym. In your example that means that ExecCallback is just an alias for the type ColumnCount -> [Text]-> [Maybe Text] -> IO (), they are interchangeable.

You could change the following lines

cb :: ColumnCount -> [Text] -> [Maybe Text] -> IO ()
cb  n cnl ct = do print $ cnl !! 1
                  return ()

to

cb :: ExecCallback
cb  n cnl ct = do print $ cnl !! 1
                  return ()

and everything would still work as is. It can make your code shorter and more readable.

One other good example is

type String = [Char]

in Prelude. I bet you normally use String instead of [Char] in most cases. But you're absolutely free to use either.

Another (completely unrelated) example is the conduit package where some type synonyms make a major difference:

type Sink i = ConduitM i Void
type Consumer i m r = forall o. ConduitM i o m r

For something that's a sink for values of any type i, Sink i seems way more readable than ConduitM i Void. Same for Consumer.