Editing Compatibility Modules (section)

= Problem statement =

Consider a very simple (and obviously fake) example. Suppose base for some terrible reason decided to provide a function which attempts to parse an <code>Int</code> and, on failure, uses a default value of 0:

<pre>module Parse where

parseInt :: String -> Int
parseInt s =
    case reads s of
        (i,""):_ -> i
        _ -> 0

-- Some other functions as well</pre>

This module makes it into base version 4.6 but then, when releasing base 4.7, we decide that the default value should really be an argument to the function instead of hard-coded to 0. Thus, our type signature changes to:

<pre>parseInt :: Int -> String -> Int</pre>

Making this change in base itself is trivial. The problem is for all the user code out there using <code>parseInt</code>. In order to allow user code to be compatible with both version 4.6 and 4.7 of base, the user must rely on techniques such as Cabal CPP macros, e.g.:

<pre>myFunc s = 5 +
#if MIN_VERSION_base(4, 7, 0)
    parseInt 0 s
#else
    parseInt s
#endif</pre>

Such conditionals are tedious and error-prone. The goal of this proposal is to simplify the life of users trying to write code that will compile against multiple GHC versions, while allowing GHC-bundled libraries to be updated in reasonable manner.