# Filenames and separate compilation

This section describes what files GHC expects to find, what files it creates, where these files are stored, and what options affect this behaviour.

Note that this section is written with hierarchical modules in mind (see ); hierarchical modules are an extension to Haskell 98 which extends the lexical syntax of module names to include a dot ‘.’. Non-hierarchical modules are thus a special case in which none of the module names contain dots.

Pathname conventions vary from system to system. In particular, the directory separator is ‘`/`’ on Unix systems and ‘`\`’ on Windows systems. In the sections that follow, we shall consistently use ‘`/`’ as the directory separator; substitute this for the appropriate character for your system.

## Haskell source files

Each Haskell source module should be placed in a file on its own.

Usually, the file should be named after the module name, replacing dots in the module name by directory separators. For example, on a Unix system, the module `A.B.C` should be placed in the file `A/B/C.hs`, relative to some base directory. If the module is not going to be imported by another module (`Main`, for example), then you are free to use any filename for it.

GHC assumes that source files are ASCII
or UTF-8
only, other encodings
are not recognised. However, invalid UTF-8 sequences will be ignored in comments, so it is possible to use other encodings such as Latin-1
, as long as the non-comment source code is ASCII only.

## Output files

When asked to compile a source file, GHC normally generates two files: an object file, and an interface file.

The object file, which normally ends in a `.o` suffix, contains the compiled code for the module.

The interface file, which normally ends in a `.hi` suffix, contains the information that GHC needs in order to compile further modules that depend on this module. It contains things like the types of exported functions, definitions of data types, and so on. It is stored in a binary format, so don't try to read one; use the `--show-iface` option instead (see ).

You should think of the object file and the interface file as a pair, since the interface file is in a sense a compiler-readable description of the contents of the object file. If the interface file and object file get out of sync for any reason, then the compiler may end up making assumptions about the object file that aren't true; trouble will almost certainly follow. For this reason, we recommend keeping object files and interface files in the same place (GHC does this by default, but it is possible to override the defaults as we'll explain shortly).

Every module has a module name defined in its source code (`module A.B.C where ...`).

The name of the object file generated by GHC is derived according to the following rules, where osuf is the object-file suffix (this can be changed with the `-osuf` option).

- If there is no `-odir` option (the default), then the object filename is derived from the source filename (ignoring the module name) by replacing the suffix with osuf.

- If `-odir` dir has been specified, then the object filename is dir/mod.osuf, where mod is the module name with dots replaced by slashes. GHC will silently create the necessary directory structure underneath dir, if it does not already exist.

The name of the interface file is derived using the same rules, except that the suffix is hisuf (`.hi` by default) instead of osuf, and the relevant options are `-hidir` and `-hisuf` instead of `-odir` and `-osuf` respectively.

For example, if GHC compiles the module `A.B.C` in the file `src/A/B/C.hs`, with no `-odir` or `-hidir` flags, the interface file will be put in `src/A/B/C.hi` and the object file in `src/A/B/C.o`.

For any module that is imported, GHC requires that the name of the module in the import statement exactly matches the name of the module in the interface file (or source file) found using the strategy specified in . This means that for most modules, the source file name should match the module name.

However, note that it is reasonable to have a module `Main` in a file named `foo.hs`, but this only works because GHC never needs to search for the interface for module `Main` (because it is never imported). It is therefore possible to have several `Main` modules in separate source files in the same directory, and GHC will not get confused.

In batch compilation mode, the name of the object file can also be overridden using the `-o` option, and the name of the interface file can be specified directly using the `-ohi` option.

## The search path

In your program, you import a module `Foo` by saying `import Foo`. In `--make` mode or GHCi, GHC will look for a source file for `Foo` and arrange to compile it first. Without `--make`, GHC will look for the interface file for `Foo`, which should have been created by an earlier compilation of `Foo`. GHC uses the same strategy in each of these cases for finding the appropriate file.

This strategy is as follows: GHC keeps a list of directories called the search path. For each of these directories, it tries appending basename`.`extension to the directory, and checks whether the file exists. The value of basename is the module name with dots replaced by the directory separator ('/' or '\', depending on the system), and extension is a source extension (`hs`, `lhs`) if we are in `--make` mode or GHCi, or hisuf otherwise.

For example, suppose the search path contains directories `d1`, `d2`, and `d3`, and we are in `--make` mode looking for the source file for a module `A.B.C`. GHC will look in `d1/A/B/C.hs`, `d1/A/B/C.lhs`, `d2/A/B/C.hs`, and so on.

The search path by default contains a single directory: . (i.e. the current directory). The following options can be used to add to or change the contents of the search path:

`-idirs`

  This flag appends a colon-separated list of `dirs` to the search path.

`-i`

  resets the search path back to nothing.

This isn't the whole story: GHC also looks for modules in pre-compiled libraries, known as packages. See the section on packages () for details.

## Redirecting the compilation output(s)

`-o` file

  GHC's compiled output normally goes into a `.hc`, `.o`, etc., file, depending on the last-run compilation phase. The option `-o file` re-directs the output of that last-run phase to file.

  Note: this “feature” can be counterintuitive: `ghc -C -o foo.o foo.hs` will put the intermediate C code in the file `foo.o`, name notwithstanding!

  This option is most often used when creating an executable file, to set the filename of the executable. For example:

         ghc -o prog --make Main

  will compile the program starting with module `Main` and put the executable in the file `prog`.

  Note: on Windows, if the result is an executable file, the extension "`.exe`" is added if the specified filename does not already have an extension. Thus

         ghc -o foo Main.hs

  will compile and link the module `Main.hs`, and put the resulting executable in `foo.exe` (not `foo`).

  If you use `ghc --make` and you don't use the `-o`, the name GHC will choose for the executable will be based on the name of the file containing the module `Main`. Note that with GHC the `Main` module doesn't have to be put in file `Main.hs`. Thus both

         ghc --make Prog

  and

         ghc --make Prog.hs

  will produce `Prog` (or `Prog.exe` if you are on Windows).
`-odir` dir

  Redirects object files to directory dir. For example:

      $ ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `uname -m`

  The object files, `Foo.o`, `Bar.o`, and `Bumble.o` would be put into a subdirectory named after the architecture of the executing machine (`x86`, `mips`, etc).

  Note that the `-odir` option does not affect where the interface files are put; use the `-hidir` option for that. In the above example, they would still be put in `parse/Foo.hi`, `parse/Bar.hi`, and `gurgle/Bumble.hi`.

`-ohi` file

  The interface output may be directed to another file `bar2/Wurble.iface` with the option `-ohi bar2/Wurble.iface` (not recommended).

  WARNING: if you redirect the interface file somewhere that GHC can't find it, then the recompilation checker may get confused (at the least, you won't get any recompilation avoidance). We recommend using a combination of `-hidir` and `-hisuf` options instead, if possible.

  To avoid generating an interface at all, you could use this option to redirect the interface into the bit bucket: `-ohi /dev/null`, for example.

`-hidir` dir

  Redirects all generated interface files into dir, instead of the default.

`-stubdir` dir

  Redirects all generated FFI stub files into dir. Stub files are generated when the Haskell source contains a `foreign export` or `foreign import "&wrapper"` declaration (see ). The `-stubdir` option behaves in exactly the same way as `-odir` and `-hidir` with respect to hierarchical modules.

`-dumpdir` dir

  Redirects all dump files into dir. Dump files are generated when `-ddump-to-file` is used with other `-ddump-*` flags.

`-outputdir` dir

  The `-outputdir` option is shorthand for the combination of `-odir`, `-hidir`, `-stubdir` and `-dumpdir`.

`-osuf` suffix
`-hisuf` suffix
`-hcsuf` suffix

  The `-osuf` suffix will change the `.o` file suffix for object files to whatever you specify. We use this when compiling libraries, so that objects for the profiling versions of the libraries don't clobber the normal ones.

  Similarly, the `-hisuf` suffix will change the `.hi` file suffix for non-system interface files (see ).

  Finally, the option `-hcsuf` suffix will change the `.hc` file suffix for compiler-generated intermediate C files.

  The `-hisuf`/`-osuf` game is particularly useful if you want to compile a program both with and without profiling, in the same directory. You can say:

      	      ghc ...

  to get the ordinary version, and

      	      ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all

  to get the profiled version.

## Keeping Intermediate Files

The following options are useful for keeping certain intermediate files around, when normally GHC would throw these away after compilation:

`-keep-hc-file`, `-keep-hc-files`

  Keep intermediate `.hc` files when doing `.hs`-to-`.o` compilations via C (NOTE: `.hc` files are only generated by unregisterised compilers).

`-keep-llvm-file`, `-keep-llvm-files`

  Keep intermediate `.ll` files when doing `.hs`-to-`.o` compilations via LLVM (NOTE: `.ll` files aren't generated when using the native code generator, you may need to use `-fllvm` to force them to be produced).

`-keep-s-file`, `-keep-s-files`

  Keep intermediate `.s` files.

`-keep-tmp-files`

  Instructs the GHC driver not to delete any of its temporary files, which it normally keeps in `/tmp` (or possibly elsewhere; see ). Running GHC with `-v` will show you what temporary files were generated along the way.

## Redirecting temporary files

`-tmpdir`

  If you have trouble because of running out of space in `/tmp` (or wherever your installation thinks temporary files should go), you may use the `-tmpdir <dir>`
  option to specify an alternate directory. For example, `-tmpdir .` says to put temporary files in the current working directory.

  Alternatively, use your `TMPDIR` environment variable.
  Set it to the name of the directory where temporary files should be put. GCC and other programs will honour the `TMPDIR` variable as well.

  Even better idea: Set the `DEFAULT_TMPDIR` make variable when building GHC, and never worry about `TMPDIR` again. (see the build documentation).

## Other options related to interface files

`-ddump-hi`

  Dumps the new interface to standard output.

`-ddump-hi-diffs`

  The compiler does not overwrite an existing `.hi` interface file if the new one is the same as the old one; this is friendly to `make`. When an interface does change, it is often enlightening to be informed. The `-ddump-hi-diffs` option will make GHC report the differences between the old and new `.hi` files.

`-ddump-minimal-imports`

  Dump to the file `M.imports` (where M is the name of the module being compiled) a "minimal" set of import declarations. The directory where the `.imports` files are created can be controlled via the `-dumpdir` option.

  You can safely replace all the import declarations in `M.hs` with those found in its respective `.imports` file. Why would you want to do that? Because the "minimal" imports (a) import everything explicitly, by name, and (b) import nothing that is not required. It can be quite painful to maintain this property by hand, so this flag is intended to reduce the labour.

`--show-iface` file

  where file is the name of an interface file, dumps the contents of that interface in a human-readable (ish) format. See .

## The recompilation checker

`-fforce-recomp`

  Turn off recompilation checking (which is on by default). Recompilation checking normally stops compilation early, leaving an existing `.o` file in place, if it can be determined that the module does not need to be recompiled.

In the olden days, GHC compared the newly-generated `.hi` file with the previous version; if they were identical, it left the old one alone and didn't change its modification date. In consequence, importers of a module with an unchanged output `.hi` file were not recompiled.

This doesn't work any more. Suppose module `C` imports module `B`, and `B` imports module `A`. So changes to module `A` might require module `C` to be recompiled, and hence when `A.hi` changes we should check whether `C` should be recompiled. However, the dependencies of `C` will only list `B.hi`, not `A.hi`, and some changes to `A` (changing the definition of a function that appears in an inlining of a function exported by `B`, say) may conceivably not change `B.hi` one jot. So now…

GHC calculates a fingerprint (in fact an MD5 hash) of each interface file, and of each declaration within the interface file. It also keeps in every interface file a list of the fingerprints of everything it used when it last compiled the file. If the source file's modification date is earlier than the `.o` file's date (i.e. the source hasn't changed since the file was last compiled), and the recompilation checking is on, GHC will be clever. It compares the fingerprints on the things it needs this time with the fingerprints on the things it needed last time (gleaned from the interface file of the module being compiled); if they are all the same it stops compiling early in the process saying “Compilation IS NOT required”. What a beautiful sight!

You can read about how all this works in the GHC commentary.

## How to compile mutually recursive modules

GHC supports the compilation of mutually recursive modules. This section explains how.

Every cycle in the module import graph must be broken by a `hs-boot` file. Suppose that modules `A.hs` and `B.hs` are Haskell source files, thus:

    module A where
        import B( TB(..) )

        newtype TA = MkTA Int

        f :: TB -> TA
        f (MkTB x) = MkTA x

    module B where
        import {-# SOURCE #-} A( TA(..) )

        data TB = MkTB !Int

        g :: TA -> TB
        g (MkTA x) = MkTB x

Here `A` imports `B`, but `B` imports `A` with a `{-# SOURCE #-}` pragma, which breaks the circular dependency. Every loop in the module import graph must be broken by a `{-# SOURCE #-}` import; or, equivalently, the module import graph must be acyclic if `{-# SOURCE #-}` imports are ignored.

For every module `A.hs` that is `{-# SOURCE #-}`-imported in this way there must exist a source file `A.hs-boot`. This file contains an abbreviated version of `A.hs`, thus:

    module A where
        newtype TA = MkTA Int

To compile these three files, issue the following commands:

      ghc -c A.hs-boot    -- Produces A.hi-boot, A.o-boot
      ghc -c B.hs         -- Consumes A.hi-boot, produces B.hi, B.o
      ghc -c A.hs	      -- Consumes B.hi, produces A.hi, A.o
      ghc -o foo A.o B.o  -- Linking the program

There are several points to note here:

- The file `A.hs-boot` is a programmer-written source file. It must live in the same directory as its parent source file `A.hs`. Currently, if you use a literate source file `A.lhs` you must also use a literate boot file, `A.lhs-boot`; and vice versa.

- A `hs-boot` file is compiled by GHC, just like a `hs` file:

        ghc -c A.hs-boot

  When a hs-boot file `A.hs-boot` is compiled, it is checked for scope and type errors. When its parent module `A.hs` is compiled, the two are compared, and an error is reported if the two are inconsistent.

- Just as compiling `A.hs` produces an interface file `A.hi`, and an object file `A.o`, so compiling `A.hs-boot` produces an interface file `A.hi-boot`, and an pseudo-object file `A.o-boot`:

  - The pseudo-object file `A.o-boot` is empty (don't link it!), but it is very useful when using a Makefile, to record when the `A.hi-boot` was last brought up to date (see ).

  - The `hi-boot` generated by compiling a `hs-boot` file is in the same machine-generated binary format as any other GHC-generated interface file (e.g. `B.hi`). You can display its contents with `ghc --show-iface`. If you specify a directory for interface files, the `-ohidir` flag, then that affects `hi-boot` files too.

- If hs-boot files are considered distinct from their parent source files, and if a `{-# SOURCE #-}` import is considered to refer to the hs-boot file, then the module import graph must have no cycles. The command `ghc -M` will report an error if a cycle is found.

- A module `M` that is `{-# SOURCE #-}`-imported in a program will usually also be ordinarily imported elsewhere. If not, `ghc --make` automatically adds `M` to the set of modules it tries to compile and link, to ensure that `M`'s implementation is included in the final program.

A hs-boot file need only contain the bare minimum of information needed to get the bootstrapping process started. For example, it doesn't need to contain declarations for everything that module `A` exports, only the things required by the module(s) that import `A` recursively.

A hs-boot file is written in a subset of Haskell:

- The module header (including the export list), and import statements, are exactly as in Haskell, and so are the scoping rules. Hence, to mention a non-Prelude type or class, you must import it.

- There must be no value declarations, but there can be type signatures for values. For example:

        double :: Int -> Int

- Fixity declarations are exactly as in Haskell.

- Vanilla type synonym declarations are exactly as in Haskell.

- Open type and data family declarations are exactly as in Haskell.

- A closed type family may optionally omit its equations, as in the following example:

        type family ClosedFam a where ..

  The `..` is meant literally -- you should write two dots in your file. Note that the `where` clause is still necessary to distinguish closed families from open ones. If you give any equations of a closed family, you must give all of them, in the same order as they appear in the accompanying Haskell file.

- A data type declaration can either be given in full, exactly as in Haskell, or it can be given abstractly, by omitting the '=' sign and everything that follows. For example:

        data T a b

  In a source program this would declare TA to have no constructors (a GHC extension: see ), but in an hi-boot file it means "I don't know or care what the constructors are". This is the most common form of data type declaration, because it's easy to get right. You can also write out the constructors but, if you do so, you must write it out precisely as in its real definition.

  If you do not write out the constructors, you may need to give a kind annotation (), to tell GHC the kind of the type variable, if it is not "*". (In source files, this is worked out from the way the type variable is used in the constructors.) For example:

        data R (x :: * -> *) y

  You cannot use `deriving` on a data type declaration; write an `instance` declaration instead.

- Class declarations is exactly as in Haskell, except that you may not put default method declarations. You can also omit all the superclasses and class methods entirely; but you must either omit them all or put them all in.

- You can include instance declarations just as in Haskell; but omit the "where" part.

- The default role for class and datatype parameters is now representational. To get another role, use a role annotation. (See .)

## Using `make`

It is reasonably straightforward to set up a `Makefile` to use with GHC, assuming you name your source files the same as your modules. Thus:

    HC      = ghc
    HC_OPTS = -cpp $(EXTRA_HC_OPTS)

    SRCS = Main.lhs Foo.lhs Bar.lhs
    OBJS = Main.o   Foo.o   Bar.o

    .SUFFIXES : .o .hs .hi .lhs .hc .s

    cool_pgm : $(OBJS)
            rm -f $@
            $(HC) -o $@ $(HC_OPTS) $(OBJS)

    # Standard suffix rules
    .o.hi:
            @:

    .lhs.o:
            $(HC) -c $< $(HC_OPTS)

    .hs.o:
            $(HC) -c $< $(HC_OPTS)

    .o-boot.hi-boot:
            @:

    .lhs-boot.o-boot:
            $(HC) -c $< $(HC_OPTS)

    .hs-boot.o-boot:
            $(HC) -c $< $(HC_OPTS)

    # Inter-module dependencies
    Foo.o Foo.hc Foo.s    : Baz.hi          # Foo imports Baz
    Main.o Main.hc Main.s : Foo.hi Baz.hi   # Main imports Foo and Baz

(Sophisticated `make` variants may achieve some of the above more elegantly. Notably, `gmake`'s pattern rules let you write the more comprehensible:

    %.o : %.lhs
            $(HC) -c $< $(HC_OPTS)

What we've shown should work with any `make`.)

Note the cheesy `.o.hi` rule: It records the dependency of the interface (`.hi`) file on the source. The rule says a `.hi` file can be made from a `.o` file by doing…nothing. Which is true.

Note that the suffix rules are all repeated twice, once for normal Haskell source files, and once for `hs-boot` files (see ).

Note also the inter-module dependencies at the end of the Makefile, which take the form

    Foo.o Foo.hc Foo.s    : Baz.hi          # Foo imports Baz

They tell `make` that if any of `Foo.o`, `Foo.hc` or `Foo.s` have an earlier modification date than `Baz.hi`, then the out-of-date file must be brought up to date. To bring it up to date, `make` looks for a rule to do so; one of the preceding suffix rules does the job nicely. These dependencies can be generated automatically by `ghc`; see

## Dependency generation

Putting inter-dependencies of the form `Foo.o : Bar.hi` into your `Makefile` by hand is rather error-prone. Don't worry, GHC has support for automatically generating the required dependencies. Add the following to your `Makefile`:

    depend :
            ghc -M $(HC_OPTS) $(SRCS)

Now, before you start compiling, and any time you change the `imports` in your program, do `make depend` before you do `make cool&lowbar;pgm`. The command `ghc -M` will append the needed dependencies to your `Makefile`.

In general, `ghc -M Foo` does the following. For each module `M` in the set `Foo` plus all its imports (transitively), it adds to the Makefile:

- A line recording the dependence of the object file on the source file.

      M.o : M.hs

  (or `M.lhs` if that is the filename you used).

- For each import declaration `import X` in `M`, a line recording the dependence of `M` on `X`:

      M.o : X.hi

- For each import declaration `import {-# SOURCE #-} X` in `M`, a line recording the dependence of `M` on `X`:

      M.o : X.hi-boot

  (See for details of `hi-boot` style interface files.)

If `M` imports multiple modules, then there will be multiple lines with `M.o` as the target.

There is no need to list all of the source files as arguments to the `ghc -M` command; `ghc` traces the dependencies, just like `ghc --make` (a new feature in GHC 6.4).

Note that `ghc -M` needs to find a source file for each module in the dependency graph, so that it can parse the import declarations and follow dependencies. Any pre-compiled modules without source files must therefore belong to a package This is a change in behaviour relative to 6.2 and earlier. .

By default, `ghc -M` generates all the dependencies, and then concatenates them onto the end of `makefile` (or `Makefile` if `makefile` doesn't exist) bracketed by the lines "`&num; DO NOT DELETE: Beginning of Haskell dependencies`" and "`&num; DO NOT DELETE: End of Haskell dependencies`". If these lines already exist in the `makefile`, then the old dependencies are deleted first.

Don't forget to use the same `-package` options on the `ghc -M` command line as you would when compiling; this enables the dependency generator to locate any imported modules that come from packages. The package modules won't be included in the dependencies generated, though (but see the `--include-pkg-deps` option below).

The dependency generation phase of GHC can take some additional options, which you may find useful.

The options which affect dependency generation are:

`-ddump-mod-cycles`

  Display a list of the cycles in the module graph. This is useful when trying to eliminate such cycles.

`-v2`

  Print a full list of the module dependencies to stdout. (This is the standard verbosity flag, so the list will also be displayed with `-v3` and `-v4`; .)

`-dep-makefile` file

  Use file as the makefile, rather than `makefile` or `Makefile`. If file doesn't exist, `mkdependHS` creates it. We often use `-dep-makefile .depend` to put the dependencies in `.depend` and then `include` the file `.depend` into `Makefile`.

`-dep-suffix <suf>`

  Make extra dependencies that declare that files with suffix `.<suf>&lowbar;<osuf>` depend on interface files with suffix `.<suf>&lowbar;hi`, or (for `&lcub;-&num; SOURCE &num;-&rcub;` imports) on `.hi-boot`. Multiple `-dep-suffix` flags are permitted. For example, `-dep-suffix a -dep-suffix b` will make dependencies for `.hs` on `.hi`, `.a&lowbar;hs` on `.a&lowbar;hi`, and `.b&lowbar;hs` on `.b&lowbar;hi`. (Useful in conjunction with NoFib "ways".)

`--exclude-module=<file>`

  Regard `<file>` as "stable"; i.e., exclude it from having dependencies on it.

`--include-pkg-deps`

  Regard modules imported from packages as unstable, i.e., generate dependencies on any imported package modules (including `Prelude`, and all other standard Haskell libraries). Dependencies are not traced recursively into packages; dependencies are only generated for home-package modules on external-package modules directly imported by the home package module. This option is normally only used by the various system libraries.

## Orphan modules and instance declarations

Haskell specifies that when compiling module M, any instance declaration in any module "below" M is visible. (Module A is "below" M if A is imported directly by M, or if A is below a module that M imports directly.) In principle, GHC must therefore read the interface files of every module below M, just in case they contain an instance declaration that matters to M. This would be a disaster in practice, so GHC tries to be clever.

In particular, if an instance declaration is in the same module as the definition of any type or class mentioned in the head of the instance declaration (the part after the “`=>`”; see ), then GHC has to visit that interface file anyway. Example:

      module A where
        instance C a => D (T a) where ...
        data T a = ...

The instance declaration is only relevant if the type T is in use, and if so, GHC will have visited A's interface file to find T's definition.

The only problem comes when a module contains an instance declaration and GHC has no other reason for visiting the module. Example:

      module Orphan where
        instance C a => D (T a) where ...
        class C a where ...

Here, neither D nor T is declared in module Orphan. We call such modules “orphan modules”. GHC identifies orphan modules, and visits the interface file of every orphan module below the module being compiled. This is usually wasted work, but there is no avoiding it. You should therefore do your best to have as few orphan modules as possible.

Functional dependencies complicate matters. Suppose we have:

      module B where
        instance E T Int where ...
        data T = ...

Is this an orphan module? Apparently not, because `T` is declared in the same module. But suppose class `E` had a functional dependency:

      module Lib where
        class E x y | y -> x where ...

Then in some importing module M, the constraint `(E a Int)` should be "improved" by setting `a = T`, even though there is no explicit mention of `T` in M.

These considerations lead to the following definition of an orphan module:

- An orphan module
  contains at least one orphan instance or at least one orphan rule.

- An instance declaration in a module M is an orphan instance if

  - The class of the instance declaration is not declared in M, and

  - Either the class has no functional dependencies, and none of the type constructors in the instance head is declared in M; or there is a functional dependency for which none of the type constructors mentioned in the non-determined part of the instance head is defined in M.

  Only the instance head counts. In the example above, it is not good enough for C's declaration to be in module A; it must be the declaration of D or T.

- A rewrite rule in a module M is an orphan rule
  if none of the variables, type constructors, or classes that are free in the left hand side of the rule are declared in M.

If you use the flag `-fwarn-orphans`, GHC will warn you if you are creating an orphan module. Like any warning, you can switch the warning off with `-fno-warn-orphans`, and `-Werror` will make the compilation fail if the warning is issued.

You can identify an orphan module by looking in its interface file, `M.hi`, using the `--show-iface` mode. If there is a `[orphan module]` on the first line, GHC considers it an orphan module.