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 Section 7.3.3, “Hierarchical Modules”); 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.
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.
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 Section 4.7.7, “Other options related to interface files”).
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 Section 4.7.3, “The search path”. 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.
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:
This isn't the whole story: GHC also looks for modules in pre-compiled libraries, known as packages. See the section on packages (Section 4.9, “ Packages ”) for details.
-o file
            
          GHC's compiled output normally goes into a
            .hc, .o, etc.,
            file, depending on the last-run compilation phase.  The
            option -o 
            re-directs the output of that last-run phase to
            filefile.
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 Section 8.2.1, “Using foreign export and foreign import ccall "wrapper" with GHC”).  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 Section 4.7.7, “Other options related to interface files”).
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.
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 Section 4.7.6, “Redirecting temporary files”).  Running GHC with
	    -v will show you what temporary files
	    were generated along the way.
-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).
-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 module being compiled) a "minimal" set of import declarations. You can safely replace all the import declarations in "M.hs" with those found in "M.imports". 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 Section 4.5, “Modes of operation”.
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.
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 Section 4.7.10, “Using make”).
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.
Type synonym declarations are exactly as in Haskell.
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 Section 7.4.1, “Data types with no constructors”), 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 (Section 7.12.4, “Explicitly-kinded quantification”), 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.
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 Section 4.7.9, “How to compile mutually recursive modules”).
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
      Section 4.7.11, “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_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 Section 4.7.9, “How to compile mutually recursive modules” 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[7].
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 "# DO NOT
        DELETE: Beginning of Haskell dependencies" and
        "# 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-cyclesDisplay a list of the cycles in the module graph. This is useful when trying to eliminate such cycles.
-v2Print 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;
	      Section 4.6, “Help and verbosity options”.)
-dep-makefile fileUse 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>_<osuf>
              depend on interface files with suffix
              .<suf>_hi, or (for
              {-# SOURCE #-}
              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_hs on
              .a_hi, and
              .b_hs on
              .b_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-depsRegard 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.
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 Section 7.6.3.2, “Relaxed rules for instance contexts”), 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.
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.