Simple debugging in Haskell

Question

I am new to Haskell. Previously I have programmed in Python and Java. When I am debugging some code I have a habit of littering it with print statements in the midd

Answer 1

I was able to create a dual personality IO / ST monad typeclass, which will print debug statements when a monadic computation is typed as IO, them when it's typed as ST. Demonstration and code here: Haskell -- dual personality IO / ST monad? .

Of course Debug.Trace is more of a swiss army knife, especially when wrapped with a useful special case,

trace2 :: Show a => [Char] -> a -> a
trace2 name x = trace (name ++ ": " ++ show x) x

which can be used like (trace2 "first arg" 3) + 4

edit

You can make this even fancier if you want source locations

{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
import Language.Haskell.TH.Syntax as TH
import Debug.Trace

withLocation :: Q Exp -> Q Exp
withLocation f = do
    let error = locationString =<< location
    appE f error
    where
        locationString :: Loc -> Q Exp
        locationString loc = do
            litE $ stringL $ formatLoc loc

formatLoc :: Loc -> String
formatLoc loc = let file = loc_filename loc
                    (line, col) = loc_start loc
                in concat [file, ":", show line, ":", show col]

trace3' (loc :: String) msg x =
    trace2 ('[' : loc ++ "] " ++ msg) x
trace3 = withLocation [| trace3' |]

then, in a separate file [from the definition above], you can write

{-# LANGUAGE TemplateHaskell #-}
tr3 x = $trace3 "hello" x

and test it out

> tr3 4
[MyFile.hs:2:9] hello: 4