Chapter 4. Using GHC
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## Chapter 4. Using GHC

## 4.1. Options overview

GHC's behaviour is controlled by options, which for historical reasons are also sometimes referred to as command-line flags or arguments. Options can be specified in three ways:

### 4.1.1. command-line arguments

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`.

### 4.1.2. command line options in source files

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).

### 4.1.3. Setting options in GHCi

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

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