Supporting alex <3.2 and >= 3.2 in the same file

[ethercrow]

It seems like one must handle AlexLastAcc like this with alex <3.2

  AlexLastAcc k input len -> ... AlexToken input len k ...

and like this with alex >= 3.2

  AlexLastAcc k input len -> ... AlexToken input len (alex_actions ! k) ...

Is there a CPP define for alex version so I can at least put this line under #ifdef?

Or maybe is there another way to write it so it works with both versions like a typeclass with instances for Int and a type of alex_action?

Problem is that stackage LTS provides 3.1.7 and stackage nightly provides 3.2.1 and I'd like very much for yi to compile with both.

[ethercrow]

The relevant line: https://github.com/yi-editor/yi/blob/master/yi-language/src/Yi/Lexer/common.hsinc#L39

Here act is either an action or an index for alex_actions.

[simonmar]

Can you do something like this in your .cabal file?

flag alex32
  default: True

if flag(alex32)
  build-depends: alex >= 3.2
  ghc-options: -DALEX_3_2
else
  build-depends: alex < 3.2

[ethercrow]

I can but this would put the burden of knowing their alex version and supplying this flag on users, who most likely don't even know what alex is.

I'll do that if there's no other option.

@snoyberg @juhp Does stack by any chance expose alex version in some #define?

[simonmar]

I can but this would put the burden of knowing their alex version and supplying this flag on users, who most likely don't even know what alex is.

No - Cabal infers the value of flags automatically, users wouldn't need to do anything.

[snoyberg]

I'm not sure this will work; build-depends doesn't play nicely with executable-only packages.

[simonmar]

If it doesn't, then you could use another package as a proxy. (a gross hack, but it would probably work)

[ethercrow]

@qnikst proposed to use TOOL_VERSION_alex defined by Cabal

[ethercrow]

I'll leave this here in case someone will have the same question.

 +#if MIN_TOOL_VERSION_alex(3,2,0)
 +      (AlexLastAcc k input'' len, _, lookahead) -> (AlexToken input'' len (alex_actions ! k), lookahead)
 +#else
        (AlexLastAcc k input'' len, _, lookahead) -> (AlexToken input'' len k, lookahead)
 +#endif