interpol

variable interpolations

Examples

The interpol pre-processor parses Haskell source file before GHC and performs variable interpolation statically. Concretely, it replaces {identifier} patterns in literal strings with show identifier. For instance,

okVal = 23
"I have {okVal} apples."

becomes

"I have 23 apples."

This works on any type that has a Show instance and is safe, in the sense that it does not disable any of GHC's normal checks (i.e. interpolating a non-existing identifier or one whose type does not have a Show instance will result in the appropriate error).

Usage

To use interpol, install the cabal package (and/or make sure that the interpol executable is in your path), and compile with the "-F -pgmF interpol" GHC options. For instance, one of the tests for this package is compiled with:

ghc -F -pgmF interpol Test/One.hs

Alternatively, you may specify the options in a GHC_OPTIONS pragma at the top of the file:

{-# OPTIONS_GHC -F -pgmF interpol #-}

Operation

The interpol pre-processor effectively replaces "\\{[A-z_][A-z0-9_]*}" with "++ show <ident> ++". So,

"I have {okVal} apples."

actually becomes

("I have " ++ show okVal ++ " apples.")

Run the pre-processor manually and check out the source for details (seriously now, this README is longer than the source).