Type inference made mechanical

Haskell uses type inference, so explicit type declarations are rarely required. This:

    import Control.Monad.Fix
    fibs = fix ((1:) . scanl (+) 1)
    main = print (take 20 fibs)

Is just as good as:

    import Control.Monad.Fix

    fibs :: [Integer]
    fibs = fix ((1:) . scanl (+) 1)

    main :: IO ()
    main = print (take 20 fibs)

Running this:

    $ runhaskell A.hs
    [1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765]

Now, once programs reach a decent size, the advantage of type declarations appears: it functions as machine-checkable documentation. So people new to your code can more quickly work out what the code is doing.

Contrast this (real world) Haskell code from a network client:

    accessorMS decompose f = withMS $ s writer ->
      let (t,k) = decompose s in f t (writer . k)

And with type annotations:

    accessorMS :: (s -> (t, t -> s))
               -> (t -> (t -> LB ()) -> LB a)
               -> ModuleT s LB a

    accessorMS decompose f = withMS $ s writer ->
      let (t,k) = decompose s in f t (writer . k)

So at least you know have some idea of what that code does.

There’s an intuition here: type declarations are good, but they can be mechanically inferred. Let’s automate that then! Here’s a quick script I use every day. It just passes your top level declaration to ghci, and asks it to infer the type. The resulting type signature is spliced back in to your code:

    #!/bin/sh

    # input is a top level .hs decls

    FILE=$*
    DECL=`cat`
    ID=`echo $DECL | sed 's/^([^ ]*).*/1/'`
    echo ":t $ID" | ghci -v0 -cpp -fglasgow-exts -w $FILE
    echo $DECL

Save this as an executable shell file in your path. Now you can call this from your editor. For example, from vim, you’d use the following .vimrc:

    :map ty :.!typeOf %

Hitting :ty while the cursor is positioned on top of a top level declaration takes your code from:

    writePS who x = withPS who (_ writer -> writer x)

to this:

    writePS :: forall p g. String -> Maybe p -> ModuleT (GlobalPrivate g p) LB ()
    writePS who x = withPS who (_ writer -> writer x)

I can’t emphasise enough how useful this is. So, steal this code and improve your productivity!