debug-trace-var

When writing print debug, we often write.

import           Debug.Trace

main :: IO ()
main = do
  let a = 1
  traceIO $ "a = " ++ show a

This is troublesome to describe the name of the variable twice.

With debug-trace-var you write variables only once.

{-# LANGUAGE QuasiQuotes #-}
import           Debug.Trace.Var

main :: IO ()
main = do
  let a = 1
  [traceVarIO|a|]

or

{-# LANGUAGE TemplateHaskell #-}
import           Debug.Trace.Var

main :: IO ()
main = let a = 1 :: Int
       in $(traceMTH 'a)

You may avoid name quotes that are confusing with character literals, Or often it may be to avoid the QuasiQuotes to destroy the syntax highlight of the text editor.

Example

{-# LANGUAGE QuasiQuotes     #-}
{-# LANGUAGE TemplateHaskell #-}

import           Debug.Trace.Var

-- > a
-- n = 1
-- s = "あ"
a :: IO ()
a = let n = 1
        s = "あ"
    in [traceVar|n|] ([traceVar|s|] return ())

-- > b
-- "n = 1
-- n = 1"
b :: String
b = let n = 1
    in [traceVarId|n|]

-- > c
-- n = 344
c :: IO ()
c = let n = 344
    in [traceStackVar|n|] (return ())

-- > d
-- n = 344
-- n = 344
d :: IO ()
d = do
  let n = 344
  [traceVarIO|n|]
  [traceVarIO|n|]

-- > e
-- n = 344
e :: IO ()
e = do
  let n = 344
  [traceVarM|n|]

-- > f
-- n = 344
f :: IO ()
f = let n = 344
    in $(traceMTH 'n)

Motivation

I wanted to use Template Haskell.

One point advice

You can use ndmitchell/debug: Haskell library for debugging