Sat, 18 Oct 2014

Haskell : A neat trick for GHCi

Just found a really nice little hack that makes working in the GHC interactive REPL a little easier and more convenient. First of all, I added the following line to my ~/.ghci file. 


  :set -DGHC_INTERACTIVE

All that line does is define a GHC_INTERACTIVE pre-processor symbol.

Then in a file that I want to load into the REPL, I need to add this to the top of the file: 


  {-# LANGUAGE CPP #-}

and then in the file I can do things like: 


  #ifdef GHC_INTERACTIVE
  import Data.Aeson.Encode.Pretty

  prettyPrint :: Value -> IO ()
  prettyPrint = LBS.putStrLn . encodePretty
  #endif

In this particular case, I'm working with some relatively large chunks of JSON and its useful to be able to pretty print them when I'm the REPL, but I have no need for that function when I compile that module into my project.

Posted at: 09:16 | Category: CodeHacking/Haskell | Permalink