GHC version check in code

Question

I'm contributing to Alex, and it obviously depends on a lot of libraries and should compile for a lot of versions.

I need to use a function that is only available from GHC 7.6.1 to handle an error better. So I want to use an #if ... to import said function, else, I'll deal with the error differently.

I've seen some:

#if __GLASGOW_HASKELL__ >= 610
import Control.Exception ( bracketOnError )
#endif

So I did:

#if __GLASGOW_HASKELL__ >= 761
import Text.Read ( readMaybe )
#endif

Thinking that 761 is an alias to GHC version 7.6.1, when I build the cabal package and try it out, the function doesn't get imported, even though I use The Glorious Glasgow Haskell Compilation System, version 7.8.4.

So after using a program to try it out, I find out that 7.8.1 identifies in __GLASGOW_HASKELL__ as 708.

{-# LANGUAGE CPP #-}
module Main where

#if __GLASGOW_HASKELL__ == 708
ver = "==708"
#else
ver = "/=708"
#endif

main = putStrLn $ ver

And running it:

$ runhaskell if.hs
==708

How can I know what value should I use for 7.6.1, or is there a better way to deal with this?

Answer 1

That's described in section 6.11.3.1 of GHC's users guide:

For version x.y.z of GHC, the value of __GLASGOW_HASKELL__ is the integer ⟨xyy⟩ (if ⟨y⟩ is a single digit, then a leading zero is added, so for example in version 6.2 of GHC, __GLASGOW_HASKELL__==602). More information in GHC version numbering policy.

So for 7.6.1, you would check __GLASGOW_HASKELL__ >= 706. The reason for this are versions like 7.10.x.

Answer 2

As Daniel Wagner indicated, the most correct way to check for package version is usually to use a Cabal MIN_VERSION macro. For example, you could use

#if MIN_VERSION_base(4,6,0)

to determine if the base package is at least version 4.6.0, which is the earliest version with the function you seek.

The base package is a little weird. It was used both by GHC and by the now-defunct Hugs and NHC implementations. Using the Cabal macro was then a more portable way to check the base version. These days, GHC is the only one using base, so the portability argument is a little less clear, but that approach also has the advantage of checking the major and minor version numbers.

Since base versions are very tightly tied to GHC versions, you can define a reasonable fall-back form of MIN_VERSION_base for compiling without Cabal, using __GLASGOW_HASKELL__ to estimate the base version. The current containers head conditionally defines such a fall-back.

Update

As of GHC 8, the compiler itself has taken over the job of defining MIN_VERSION macros. This is great, because you now get to use these macros whether or not you build with Cabal. No more ugly approximations!