The Glasgow Haskell Compiler User's Guide, Version 6.2
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# 4.4. Modes of operation

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.

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.

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 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.

ghc –M

  Dependency-generation mode. In this mode, GHC can be used to generate dependency information suitable for use in a `Makefile`. See Section 4.6.9.1.

ghc ––mk-dll

  DLL-creation mode (Windows only). See Section 11.5.3.

## 4.4.1. Using ghc `––make`

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 It will then attempt to compile each module which is out of date, and finally if there is a `Main` module, the program will also be linked into an executable.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 `Makefile`s are:

- GHC doesn't have to be restarted for each compilation, which means it can cache information between compilations. Compiling a muli-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 a `Makefile`.

- 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` pragma (see Section 4.1.2).

If the program needs to be linked with additional objects (say, some auxilliary 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).

## 4.4.2. Expression evaluation mode

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

## 4.4.3. Batch compiler mode

In this 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 for more details.

Note: C pre-processing is optional, the `-cpp` flag turns it on. See Section 4.10.3 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.

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