interpol

variable interpolations

Examples

The interpol preprocessor 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).

Installation

This package is on Hackage. To install it, run:

cabal update
cabal install interpol

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 #-}

Note that, unless you use this latter pragma, ghc-mod and other flymake-based Emacs modes will probably complain about unused variables.

Operation

The interpol preprocessor effectively does two things:

1. it adds an import declaration for Text.Interpol, in order to bring the (^-^) operator into scope, and

2. it replaces any occurrence of "\\{[A-z_][A-z0-9_]*}" in string literals with "^-^ <ident> ^-^".

So,

"I have {okVal} apples."

actually becomes

("I have " ^-^ okVal ^-^ " apples.")

The (^-^) operator is a smarter version of (++): it shows its second argument before appending, but only if it is not already a String (i.e. it does not quote String values when interpolating).

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