GHC's behaviour is firstly controlled by a mode flag. Only one of these flags may be given, but it does not necessarily need to be the first option on the command-line. The available modes are:
ghc ––interactive
Interactive mode, which is also available as ghci. Interactive mode is described in more detail in Chapter 3, Using GHCi.
ghc ––make
In this mode, GHC will build a multi-module Haskell
program automatically, figuring out dependencies for itself.
If you have a straightforward Haskell program, this is
likely to be much easier, and faster, than using
make. Make mode is described in Section 4.4.1, “Using ghc ––make”.
ghc –e expr
Expression-evaluation mode. This is very similar to
interactive mode, except that there is a single expression
to evaluate (expr) which is given
on the command line. See Section 4.4.2, “Expression evaluation mode” for
more details.
ghc [[-E] | [-C] | [-S] | [-c]]
This is the traditional batch-compiler mode, in which GHC can compile source files one at a time, or link objects together into an executable. This mode also applies if there is no other mode flag specified on the command line, in which case it means that the specified files should be compiled and then linked to form a program. See Section 4.4.3, “Batch compiler mode”.
ghc –M
Dependency-generation mode. In this mode, GHC can be
used to generate dependency information suitable for use in
a Makefile. See Section 4.6.11, “Dependency generation”.
ghc ––mk-dll
DLL-creation mode (Windows only). See Section 11.5.1, “Creating a DLL”.
When given the ––make option,
GHC will build a multi-module Haskell program by following
dependencies from a single root module (usually
Main). For example, if your
Main module is in a file called
Main.hs, you could compile and link the
program like this:
ghc ––make Main.hs
The command line may contain any number of source file
names or module names; GHC will figure out all the modules in
the program by following the imports from these initial modules.
It will then attempt to compile each module which is out of
date, and finally if there is a Main module,
the program will also be linked into an executable.
The main advantages to using ghc
––make over traditional
Makefiles are:
GHC doesn't have to be restarted for each compilation,
which means it can cache information between compilations.
Compiling a multi-module program with ghc
––make can be up to twice as fast as
running ghc individually on each source
file.
You don't have to write aMakefile.
GHC re-calculates the dependencies each time it is invoked, so the dependencies never get out of sync with the source.
Any of the command-line options described in the rest of
this chapter can be used with
––make, but note that any options
you give on the command line will apply to all the source files
compiled, so if you want any options to apply to a single source
file only, you'll need to use an OPTIONS_GHC
pragma (see Section 4.1.2, “command line options in source files”).
If the program needs to be linked with additional objects (say, some auxiliary C code), then the object files can be given on the command line and GHC will include them when linking the executable.
Note that GHC can only follow dependencies if it has the source file available, so if your program includes a module for which there is no source file, even if you have an object and an interface file for the module, then GHC will complain. The exception to this rule is for package modules, which may or may not have source files.
The source files for the program don't all need to be in
the same directory; the -i option can be used
to add directories to the search path (see Section 4.6.3, “The search path”).
This mode is very similar to interactive mode, except that
there is a single expression to evaluate which is specified on
the command line as an argument to the -e
option:
ghc -e expr
Haskell source files may be named on the command line, and they will be loaded exactly as in interactive mode. The expression is evaluated in the context of the loaded modules.
For example, to load and run a Haskell program containing
a module Main, we might say
ghc -e Main.main Main.hs
or we can just use this mode to evaluate expressions in
the context of the Prelude:
$ ghc -e "interact (unlines.map reverse.lines)" hello olleh
In batch mode, GHC will compile one or more source files given on the command line.
The first phase to run is determined by each input-file suffix, and the last phase is determined by a flag. If no relevant flag is present, then go all the way through linking. This table summarises:
| Phase of the compilation system | Suffix saying “start here” | Flag saying “stop after” | (suffix of) output file |
|---|---|---|---|
| literate pre-processor | .lhs | - | .hs |
| C pre-processor (opt.) | .hs (with
-cpp) | -E | .hspp |
| Haskell compiler | .hs | -C, -S | .hc, .s |
| C compiler (opt.) | .hc or .c | -S | .s |
| assembler | .s | -c | .o |
| linker | other | - | a.out |
Thus, a common invocation would be:
ghc -c Foo.hs
to compile the Haskell source file
Foo.hs to an object file
Foo.o.
Note: What the Haskell compiler proper produces depends on whether a native-code generator is used (producing assembly language) or not (producing C). See Section 4.10.6, “Options affecting code generation” for more details.
Note: C pre-processing is optional, the
-cpp
flag turns it on. See Section 4.10.3, “Options affecting the C pre-processor” for more
details.
Note: The option -E runs just the pre-processing passes
of the compiler, dumping the result in a file. Note that this
differs from the previous behaviour of dumping the file to
standard output.