4.6. Filenames and separate compilation

4.6.1. Haskell source files

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

The file should usually be named after the module name, by 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. GHC's behaviour if this rule is not followed is fully defined by the following section (Section 4.6.2).

4.6.2. 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 (or .obj if you're on Windows), 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 4.6.7).

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 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.

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 4.6.3. 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 overriden using the -o option, and the name of the interface file can be specified directly using the -ohi option.

4.6.3. 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 and 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 4.8), for details.

4.6.4. Redirecting the compilation output(s)

4.6.5. Keeping Intermediate Files

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

4.6.6. Redirecting temporary files

4.6.7. Other options related to interface files

4.6.8. The recompilation checker

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 keeps a version number on each interface file, and on each type signature within the interface file. It also keeps in every interface file a list of the version numbers 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 reompilation checking is on, GHC will be clever. It compares the version numbers on the things it needs this time with the version numbers 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 rather early in the process saying “Compilation IS NOT required”. What a beautiful sight!

Patrick Sansom had a workshop paper about how all this is done (though the details have changed quite a bit). Ask him if you want a copy.

4.6.9. 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)

# 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 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.

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

4.6.10. How to compile mutually recursive modules

Currently, the compiler does not have proper support for dealing with mutually recursive modules:

module A where

import B

newtype TA = MkTA Int

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

import A

data TB = MkTB !Int

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

When compiling either module A and B, the compiler will try (in vain) to look for the interface file of the other. So, to get mutually recursive modules off the ground, you need to hand write an interface file for A or B, so as to break the loop. These hand-written interface files are called hi-boot files, and are placed in a file called <module>.hi-boot. To import from an hi-boot file instead of the standard .hi file, use the following syntax in the importing module:

import {-# SOURCE #-} A

The hand-written interface 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 that imports A recursively.

For the example at hand, the boot interface file for A would look like the following:

module A where
newtype TA = MkTA GHC.Base.Int

The syntax is similar to a normal Haskell source file, but with some important differences:

• Non-local entities must be qualified with their original defining module. Qualifying by a module which just re-exports the entity won't do. In particular, most Prelude entities aren't actually defined in the Prelude (see for example GHC.Base.Int in the above example). HINT: to find out the fully-qualified name for entities in the Prelude (or anywhere for that matter), try using GHCi's :info command, eg.

  Prelude> :m -Prelude > :i IO.IO -- GHC.IOBase.IO is a type constructor newtype GHC.IOBase.IO a ...

• Only data, type, newtype, class, and type signature declarations may be included. You cannot declare instances or derive them automatically.

Notice that we only put the declaration for the newtype TA in the hi-boot file, not the signature for f, since f isn't used by B.

If you want an hi-boot file to export a data type, but you don't want to give its constructors (because the constructors aren't used by the SOURCE-importing module), you can write simply:

module A where
data TA

(You must write all the type parameters, but leave out the '=' and everything that follows it.)

4.6.11. 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, 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'', defined thus:

• 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 none of the type constructors or classes mentioned in the instance head (the part after the ``=>'') are declared 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.

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.

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