Development.Placeholders

This module defines placeholders that you can use while coding to allow incomplete code to compile. They work just like undefined, but with improved error messages and compile-time warnings.

Example

 {-# 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.

Placeholders

notImplemented :: Q Exp

Indicates that this piece of code has not yet been implemented.

$notImplemented = $(placeholder "Unimplemented feature")

todo :: String -> Q Exp

Indicates unimplemented code or a known bug with a custom message.

$(todo msg) = $(placeholder ("TODO: " ++ msg))

placeholder :: String -> Q Exp

Generates an expression of any type that, if evaluated at runtime will throw a PlaceholderException. It is therefore similar to error, except that the source location is automatically included. Also, a warning is generated at compile time so you won't forget to replace placeholders before packaging your code.

placeholderNoWarning :: String -> Q Exp

Similar to placeholder, but does not generate a compiler warning. Use with care!

Exceptions

data PlaceholderException

Thrown when attempting to evaluate a placeholder at runtime.

Constructors

PlaceholderException String

Instances

Show PlaceholderException

Typeable PlaceholderException

Exception PlaceholderException