An invocation of GHC takes the following form:

ghc [argument...]

command-line arguments are either options or file names.

command-line options begin with -. They may not be grouped: -vO is different from -v -O. Options need not precede filenames: e.g., ghc *.o -o foo. All options are processed and then applied to all files; you cannot, for example, invoke ghc -c -O1 Foo.hs -O2 Bar.hs to apply different optimisation levels to the files Foo.hs and Bar.hs.

Sometimes it is useful to make the connection between a source file and the command-line options it requires quite tight. For instance, if a Haskell source file uses GHC extensions, it will always need to be compiled with the -fglasgow-exts option. Rather than maintaining the list of per-file options in a Makefile, it is possible to do this directly in the source file using the OPTIONS_GHC pragma :

{-# OPTIONS_GHC -fglasgow-exts #-}
module X where
...

OPTIONS_GHC pragmas are only looked for at the top of your source files, upto the first (non-literate,non-empty) line not containing OPTIONS_GHC. Multiple OPTIONS_GHC pragmas are recognised. Do not put comments before, or on the same line as, the OPTIONS_GHC pragma.

Note that your command shell does not get to the source file options, they are just included literally in the array of command-line arguments the compiler maintains internally, so you'll be desperately disappointed if you try to glob etc. inside OPTIONS_GHC.

NOTE: the contents of OPTIONS_GHC are prepended to the command-line options, so you do have the ability to override OPTIONS_GHC settings via the command line.

It is not recommended to move all the contents of your Makefiles into your source files, but in some circumstances, the OPTIONS_GHC pragma is the Right Thing. (If you use -keep-hc-file-too and have OPTION flags in your module, the OPTIONS_GHC will get put into the generated .hc file).

Options may also be modified from within GHCi, using the :set command. See Section 3.7, “The :set command” for more details.