Placeholders for use while developing Haskell code

Version on this page:0.1
LTS Haskell 13.22:0.1
Stackage Nightly 2019-05-23:0.1
Latest on Hackage:0.1

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BSD-3-Clause licensed by Andreas Hammar
Maintained by Andreas Hammar

Module documentation for 0.1

There are no documented modules for this package.


While working on some Haskell code, it is often useful to develop incrementally by inserting undefined as a placeholder for missing code.

This approach has a couple of drawbacks.

  • If you have several occurrences of undefined in your code, it can be hard to track down the one reponsible for the error at run-time.

  • It is too easy to forget to replace undefined with the proper code, which might cause unexpected errors.

This library provides placeholders that produce better messages when evaluated at run-time and also generate compile-time warnings so that they do not get forgotten so easily.


{-# LANGUAGE TemplateHaskell #-}

import Development.Placeholders

theUltimateAnswer :: Int
theUltimateAnswer = $notImplemented

main = do
    putStrLn "The ultimate answer:"
    print theUltimateAnswer

This will compile with a warning about the unimplemented function:

$ ghc --make Simple.hs
Simple.hs:6:21: Unimplemented feature

At runtime, an exception will be thrown when the placeholder is evaluated, indicating the location of the placeholder.

$ ./Simple
The ultimate answer:
Simple: PlaceholderExcption "Unimplemented feature at Simple.hs:6:21"

If compiled with the GHC flag -Werror, the warning will get turned into an error and compilation will fail. -Werror can therefore be used to verify that you haven’t left any unintended placeholders behind.

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