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 deliberately uses name shadowing, it should be compiled with the -fno-warn-name-shadowing 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 -fno-warn-name-shadowing #-}
module X where
...

OPTIONS_GHC is a file-header pragma (see Section�7.20, “Pragmas”).

Only dynamic flags can be used in an OPTIONS_GHC pragma (see Section�4.3, “Static, Dynamic, and Mode options”).

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 appended to the command-line options, so options given in the source file override those given on 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 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�2.8, “The :set and :seti commands” for more details.