5.1. Using GHC¶

5.1.1. Getting started: compiling programs¶

In this chapter you’ll find a complete reference to the GHC command-line syntax, including all 400+ flags. It’s a large and complex system, and there are lots of details, so it can be quite hard to figure out how to get started. With that in mind, this introductory section provides a quick introduction to the basic usage of GHC for compiling a Haskell program, before the following sections dive into the full syntax.

Let’s create a Hello World program, and compile and run it. First, create a file hello.hs containing the Haskell code:

main = putStrLn "Hello, World!"

To compile the program, use GHC like this:

$ ghc hello.hs

(where $ represents the prompt: don’t type it). GHC will compile the source file hello.hs, producing an object file hello.o and an interface file hello.hi, and then it will link the object file to the libraries that come with GHC to produce an executable called hello on Unix/Linux/Mac, or hello.exe on Windows.

By default GHC will be very quiet about what it is doing, only printing error messages. If you want to see in more detail what’s going on behind the scenes, add -v to the command line.

Then we can run the program like this:

$ ./hello
Hello World!

If your program contains multiple modules, then you only need to tell GHC the name of the source file containing the Main module, and GHC will examine the import declarations to find the other modules that make up the program and find their source files. This means that, with the exception of the Main module, every source file should be named after the module name that it contains (with dots replaced by directory separators). For example, the module Data.Person would be in the file Data/Person.hs on Unix/Linux/Mac, or Data\Person.hs on Windows.

5.1.2. Options overview¶

GHC’s behaviour is controlled by options, which for historical reasons are also sometimes referred to as command-line flags or arguments. Options can be specified in three ways:

5.1.2.1. Command-line arguments¶

An invocation of GHC takes the following form:

ghc [argument...]

Command-line arguments are either options or file names.

Command-line options begin with -. They may not be grouped: -vO is different from -v -O. Options need not precede filenames: e.g., ghc *.o -o foo. All options are processed and then applied to all files; you cannot, for example, invoke ghc -c -O1 Foo.hs -O2 Bar.hs to apply different optimisation levels to the files Foo.hs and Bar.hs.

In addition to passing arguments via the command-line, arguments can be passed via GNU-style response files. For instance,

$ cat response-file
-O1
Hello.hs
-o Hello
$ ghc @response-file

Note

Note that command-line options are order-dependent, with arguments being evaluated from left-to-right. This can have seemingly strange effects in the presence of flag implication. For instance, consider -fno-specialise and -O1 (which implies -fspecialise). These two command lines mean very different things:

-fno-specialise -O1

-fspecialise will be enabled as the -fno-specialise is overridden by the -O1.

-O1 -fno-specialise

-fspecialise will not be enabled, since the -fno-specialise overrides the -fspecialise implied by -O1.

5.1.2.2. Command line options in source files¶

Sometimes it is useful to make the connection between a source file and the command-line options it requires quite tight. For instance, if a Haskell source file deliberately uses name shadowing, it should be compiled with the -Wno-name-shadowing option. Rather than maintaining the list of per-file options in a Makefile, it is possible to do this directly in the source file using the OPTIONS_GHC pragma

{-# OPTIONS_GHC -Wno-name-shadowing #-}
module X where
...

OPTIONS_GHC is a file-header pragma (see OPTIONS_GHC pragma).

Only dynamic flags can be used in an OPTIONS_GHC pragma (see Dynamic and Mode options).

Note that your command shell does not get to the source file options, they are just included literally in the array of command-line arguments the compiler maintains internally, so you’ll be desperately disappointed if you try to glob etc. inside OPTIONS_GHC.

Note

The contents of OPTIONS_GHC are appended to the command-line options, so options given in the source file override those given on the command-line.

It is not recommended to move all the contents of your Makefiles into your source files, but in some circumstances, the OPTIONS_GHC pragma is the Right Thing. (If you use -keep-hc-file and have OPTION flags in your module, the OPTIONS_GHC will get put into the generated .hc file).

5.1.2.3. Setting options in GHCi¶

Options may also be modified from within GHCi, using the :set command.

5.1.3. Dynamic and Mode options¶

Each of GHC’s command line options is classified as dynamic or mode:

Mode: A mode may be used on the command line only. You can pass only one mode flag. For example, --make or -E. The available modes are listed in Modes of operation.

Dynamic: A dynamic flag may be used on the command line, in a OPTIONS_GHC pragma in a source file, or set using :set in GHCi.

The flag reference tables (Flag reference) lists the status of each flag.

5.1.4. Meaningful file suffixes¶

File names with “meaningful” suffixes (e.g., .lhs or .o) cause the “right thing” to happen to those files.

.hs: A Haskell module.
.lhs: A “literate Haskell” module.
.hspp: A file created by the preprocessor.
.hi: A Haskell interface file, probably compiler-generated.
.hie: An extended Haskell interface file, produced by the Haskell compiler.
.hc: Intermediate C file produced by the Haskell compiler.
.c: A C file not produced by the Haskell compiler.
.ll: An llvm-intermediate-language source file, usually produced by the compiler.
.bc: An llvm-intermediate-language bitcode file, usually produced by the compiler.
.s: An assembly-language source file, usually produced by the compiler.
.o: An object file, produced by an assembler.

Files with other suffixes (or without suffixes) are passed straight to the linker.

5.1.5. 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. For instance,

$ ghc Main.hs --make -o my-application

If no mode flag is present, then GHC will enter --make mode (Using ghc --make) if there are any Haskell source files given on the command line, or else it will link the objects named on the command line to produce an executable.

The available mode flags are:

--interactive¶: Interactive mode, which is also available as ghci. Interactive mode is described in more detail in Using GHCi.

--run ⟨file⟩¶: Run a script’s main entry-point. Similar to runghc/runhaskell this will by default use the bytecode interpreter. If the command-line contains a -- argument then all arguments that follow will be passed to the script. All arguments that precede -- are interpreted as GHC arguments.

--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 Using ghc --make.

This mode is the default if there are any Haskell source files mentioned on the command line, and in this case the --make option can be omitted.

-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. This flag may be given multiple times, in which case each expression is evaluated sequentially. See Expression evaluation mode for more details.

-E¶: Stop after preprocessing (.hspp file)

-C¶: Stop after generating C (.hc file)

-S¶: Stop after generating assembly (.s file)

-c¶

Stop after generating object (.o) file

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. See Batch compiler mode.

--merge-objs¶: Merge a set of static object files into a library optimised for loading in GHCi. See Building GHCi libraries.

-M¶: Dependency-generation mode. In this mode, GHC can be used to generate dependency information suitable for use in a Makefile. See Dependency generation.

--frontend ⟨module⟩¶: Run GHC using the given frontend plugin. See Frontend plugins for details.

-shared¶: Create a shared object (or, on Windows, DLL). See Creating a DLL.

--help¶
-?¶: Cause GHC to spew a long usage message to standard output and then exit.

--show-iface ⟨file⟩¶: Read the interface in ⟨file⟩ and dump it as text to stdout. For example ghc --show-iface M.hi.

--supported-extensions¶
--supported-languages¶: Print the supported language extensions.

--show-options¶: Print the supported command line options. This flag can be used for autocompletion in a shell.

--info¶: Print information about the compiler.

--version¶
-V¶: Print a one-line string including GHC’s version number.

--numeric-version¶: Print GHC’s numeric version number only.

--print-booter-version¶: Print the numeric version of the GHC binary used to bootstrap the build of this compiler.

--print-build-platform¶: Print the target string of the build platform, on which GHC was built, as generated by GNU Autotools. The format is cpu-manufacturer-operating_system-(kernel), e.g., x86_64-unknown-linux.

--print-c-compiler-flags¶: List the flags passed to the C compiler during GHC build.

--print-c-compiler-link-flags¶: List the flags passed to the C compiler for the linking step during GHC build.

--print-debug-on¶: Print True if GHC was built with -DDebug flag. This enables assertions and extra debug code. The flag can be set in GhcStage1HcOpts and/or GhcStage2HcOpts and is automatically set for devel1 and devel2 build flavors.

--print-global-package-db¶: Print the path to GHC’s global package database directory. A package database stores details about installed packages as a directory containing a file for each package. This flag prints the path to the global database shipped with GHC, and looks something like /usr/lib/ghc/package.conf.d on Unix. There may be other package databases, e.g., the user package databse. For more details see Package Databases.

--print-have-interpreter¶: Print YES if GHC was compiled to include the interpreter, NO otherwise. If this GHC does not have the interpreter included, running it in interactive mode (see --interactive) will throw an error. This only pertains the use of GHC interactively, not any separate GHCi binaries (see Using GHCi).

--print-have-native-code-generator¶: Print YES if native code generator supports the target platform, NO otherwise. (See Native Code Generator (-fasm))

--print-host-platform¶: Print the target string of the host platform, i.e., the one on which GHC is supposed to run, as generated by GNU Autotools. The format is cpu-manufacturer-operating_system-(kernel), e.g., x86_64-unknown-linux.

--print-leading-underscore¶: Print YES if GHC was compiled to use symbols with leading underscores in object files, NO otherwise. This is usually atarget platform dependent.

--print-libdir¶: Print the path to GHC’s library directory. This is the top of the directory tree containing GHC’s libraries, interfaces, and include files (usually something like /usr/local/lib/ghc-5.04 on Unix). This is the value of $libdir in the package configuration file (see Packages).

--print-ld-flags¶: Print linke flags used to compile GHC.

--print-object-splitting-supported¶: Print YES if GHC was compiled with support for splitting generated object files into smaller objects, NO otherwise. This feature uses platform specific techniques and may not be available on all platforms. See -split-objs for details.

--print-project-git-commit-id¶: Print the Git commit id from which this GHC was built. This can be used to trace the current binary back to a specific revision, which is especially useful during development on GHC itself. It is set by the configure script.

--print-project-version¶: Print the version set in the configure script during build. This is simply the GHC version.

--print-rts-ways¶

Packages, like the Runtime System, can be built in a number of ways: - profiling - with profiling support - dynamic - with dynamic linking - logging - RTS event logging - threaded - mulithreaded RTS - debug - RTS with debug information

Various combinations of these flavours are possible.

--print-stage¶

GHC is built using GHC itself and this build happens in stages, which are numbered.

Stage 0 is the GHC you have installed. The “GHC you have installed” is also called “the bootstrap compiler”.
Stage 1 is the first GHC we build, using stage 0. Stage 1 is then used to build the packages.
Stage 2 is the second GHC we build, using stage 1. This is the one we normally install when you say make install.
Stage 3 is optional, but is sometimes built to test stage 2.

Stage 1 does not support interactive execution (GHCi) and Template Haskell.

--print-support-smp¶: Print YES if GHC was built with multiporcessor support, NO otherwise.

--print-tables-next-to-code¶: Print YES if GHC was built with the flag --enable-tables-next-to-code, NO otherwise. This option is on by default, as it generates a more efficient code layout.

--print-target-platform¶: Print the target string of the target platform, i.e., the one on which generated binaries will run, as generated by GNU Autotools. The format is cpu-manufacturer-operating_system-(kernel), e.g., x86_64-unknown-linux.

--print-unregisterised¶: Print YES if this GHC was built in unregisterised mode, NO otherwise. “Unregisterised” means that GHC will disable most platform-specific tricks and optimisations. Only the LLVM and C code generators will be available. See Unregisterised compilation for more details.

5.1.5.1. Using `ghc` `--make`¶

In this mode, GHC will build a multi-module Haskell program by following dependencies from one or more root modules (usually just 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

In fact, GHC enters make mode automatically if there are any Haskell source files on the command line and no other mode is specified, so in this case we could just type

ghc Main.hs

Any number of source file names or module names may be specified; 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 a Makefile.
GHC re-calculates the dependencies each time it is invoked, so the dependencies never get out of sync with the source.
Using the -j[⟨n⟩] flag, you can compile modules in parallel. Specify -j ⟨n⟩ to compile ⟨n⟩ jobs in parallel. If ⟨n⟩ is omitted, then it defaults to the number of processors.

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 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.

For backward compatibility with existing make scripts, when used in combination with -c, the linking phase is omitted (same as --make -no-link).

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 The search path).

-j[⟨n⟩]¶: Perform compilation in parallel when possible. GHC will use up to ⟨N⟩ threads during compilation. If N is omitted, then it defaults to the number of processors. Note that compilation of a module may not begin until its dependencies have been built.

5.1.5.2. GHC Jobserver Protocol¶

The GHC Jobserver Protocol was specified in GHC proposal #540.

This protocol allows a server to dynamically invoke many instances of a client process, while restricting all of those instances to use no more than <n> capabilities. This is achieved by coordination over a system semaphore (either a POSIX semaphore in the case of Linux and Darwin, or a Win32 semaphore in the case of Windows platforms).

There are two kinds of participants in the GHC Jobserver protocol:

The jobserver creates a system semaphore with a certain number of available tokens.

Each time the jobserver wants to spawn a new jobclient subprocess, it must first acquire a single token from the semaphore, before spawning the subprocess. This token must be released once the subprocess terminates.

Once work is finished, the jobserver must destroy the semaphore it created.
A jobclient is a subprocess spawned by the jobserver or another jobclient.

Each jobclient starts with one available token (its implicit token, which was acquired by the parent which spawned it), and can request more tokens through the Jobserver Protocol by waiting on the semaphore.

Each time a jobclient wants to spawn a new jobclient subprocess, it must pass on a single token to the child jobclient. This token can either be the jobclient’s implicit token, or another token which the jobclient acquired from the semaphore.

Each jobclient must release exactly as many tokens as it has acquired from the semaphore (this does not include the implicit tokens).

GHC itself acts as a jobclient which can be enabled by using the flag -jsem.

-jsem¶

Perform compilation in parallel when possible, coordinating with other processes through the semaphore ⟨sem⟩ (specified as a string). Error if the semaphore doesn’t exist.

Use of -jsem will override use of :ghc-flag:-j[⟨n⟩], and vice-versa.

5.1.5.3. Multiple Home Units¶

The compiler also has support for building multiple units in a single compiler invocation. In modern projects it is common to work on multiple interdependent packages at once, using the support for multiple home units you can load all these local packages into one ghc session and quickly get feedback about how changes affect other dependent packages.

In order to specify multiple units, the -unit @⟨filename⟩ is given multiple times with a response file containing the arguments for each unit. The response file contains a newline separated list of arguments.

ghc -unit @unitA -unit @unitB

where the unitA response file contains the normal arguments that you would pass to --make mode.

-this-unit-id a-0.1.0.0
-i
-isrc
A1
A2
...

Then when the compiler starts in --make mode it will compile both units a and b.

There is also very basic support for multiple home units in GHCi, at the moment you can start a GHCi session with multiple units but only the :reload is supported.

-unit @⟨filename⟩¶: This option is passed multiple times to inform the compiler about all the home units which it will compile. The options for each unit are supplied in a response file which contains a newline separated list of normal arguments.

There are a few extra flags which have been introduced to make working with multiple units easier.

-working-dir ⟨dir⟩¶

It is common to assume that a package is compiled in the directory where its cabal file resides. Thus, all paths used in the compiler are assumed to be relative to this directory. When there are multiple home units the compiler is often not operating in the standard directory and instead where the cabal.project file is located. In this case the -working-dir option can be passed which specifies the path from the current directory to the directory the unit assumes to be its root, normally the directory which contains the cabal file.

When the flag is passed, any relative paths used by the compiler are offset by the working directory. Notably this includes -i and -I⟨dir⟩ flags.

This option can also be queried by the getPackageRoot Template Haskell function. It is intended to be used with helper functions such as makeRelativeToProject which make relative filepaths relative to the compilation directory rather than the directory which contains the .cabal file.

-this-package-name ⟨unit-id⟩¶

This flag papers over the awkward interaction of the PackageImports and multiple home units. When using PackageImports you can specify the name of the package in an import to disambiguate between modules which appear in multiple packages with the same name.

This flag allows a home unit to be given a package name so that you can also disambiguate between multiple home units which provide modules with the same name.

-hidden-module ⟨module name⟩¶

This flag can be supplied multiple times in order to specify which modules in a home unit should not be visible outside of the unit it belongs to.

The main use of this flag is to be able to recreate the difference between an exposed and hidden module for installed packages.

-reexported-module ⟨module name⟩¶

This flag can be supplied multiple times in order to specify which modules are not defined in a unit but should be reexported. The effect is that other units will see this module as if it was defined in this unit.

The use of this flag is to be able to replicate the reexported modules feature of packages with multiple home units.

5.1.5.3.1. The home unit closure requirement¶

There is one very important closure property which you must ensure when using multiple home units.

Any external unit must not depend on any home unit.

This closure property is checked by the compiler but it’s up to the tool invoking GHC to ensure that the supplied list of home units obeys this invariant.

For example, if we have three units, p, q and r, where p depends on q and q depends on r, then the closure property states that if we load p and r as home units then we must also load q, because q depends on the home unit r and we need q because p depends on it.

5.1.5.4. 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

5.1.5.5. Batch compiler mode¶

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 to 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 what backend code generator is used. See GHC Backends for more details.

Note

Pre-processing is optional, the -cpp flag turns it on. See 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

The option -C is only available when GHC is built in unregisterised mode. See Unregisterised compilation for more details.

5.1.5.5.1. Overriding the default behaviour for a file¶

As described above, the way in which a file is processed by GHC depends on its suffix. This behaviour can be overridden using the -x ⟨suffix⟩ option:

-x ⟨suffix⟩¶: Causes all files following this option on the command line to be processed as if they had the suffix ⟨suffix⟩. For example, to compile a Haskell module in the file M.my-hs, use ghc -c -x hs M.my-hs.

5.1.6. Verbosity options¶

See also the --help, --version, --numeric-version, and --print-libdir modes in Modes of operation.

-v¶

The -v option makes GHC verbose: it reports its version number and shows (on stderr) exactly how it invokes each phase of the compilation system. Moreover, it passes the -v flag to most phases; each reports its version number (and possibly some other information).

Please, oh please, use the -v option when reporting bugs! Knowing that you ran the right bits in the right order is always the first thing we want to verify.

-v⟨n⟩¶

To provide more control over the compiler’s verbosity, the -v flag takes an optional numeric argument. Specifying -v on its own is equivalent to -v3, and the other levels have the following meanings:

-v0: Disable all non-essential messages (this is the default).
-v1: Minimal verbosity: print one line per compilation (this is the default when --make or --interactive is on).
-v2: Print the name of each compilation phase as it is executed. (equivalent to -dshow-passes).
-v3: The same as -v2, except that in addition the full command line (if appropriate) for each compilation phase is also printed.
-v4: The same as -v3 except that the intermediate program representation after each compilation phase is also printed (excluding preprocessed and C/assembly files).

-fprint-potential-instances¶: When GHC can’t find an instance for a class, it displays a short list of some of the instances it knows about. With this flag it prints all the instances it knows about.

-fhide-source-paths¶: Starting with minimal verbosity (-v1, see -v), GHC displays the name, the source path and the target path of each compiled module. This flag can be used to reduce GHC’s output by hiding source paths and target paths.

The following flags control the way in which GHC displays types in error messages and in GHCi:

-fprint-unicode-syntax¶

When enabled GHC prints type signatures using the unicode symbols from the UnicodeSyntax extension. For instance,

ghci> :set -fprint-unicode-syntax
ghci> :t +v (>>)
(>>) ∷ Monad m ⇒ ∀ a b. m a → m b → m b

-fprint-explicit-foralls¶

Using -fprint-explicit-foralls makes GHC print explicit forall quantification at the top level of a type; normally this is suppressed. For example, in GHCi:

ghci> let f x = x
ghci> :t f
f :: a -> a
ghci> :set -fprint-explicit-foralls
ghci> :t f
f :: forall a. a -> a

However, regardless of the flag setting, the quantifiers are printed under these circumstances:

For nested foralls, e.g.

ghci> :t GHC.ST.runST
GHC.ST.runST :: (forall s. GHC.ST.ST s a) -> a

If any of the quantified type variables has a kind that mentions a kind variable, e.g.

ghci> :i Data.Type.Equality.sym
Data.Type.Equality.sym ::
  forall k (a :: k) (b :: k).
  (a Data.Type.Equality.:~: b) -> b Data.Type.Equality.:~: a
        -- Defined in Data.Type.Equality

-fprint-explicit-kinds¶

Using -fprint-explicit-kinds makes GHC print kind arguments in types, which are normally suppressed. This can be important when you are using kind polymorphism. For example:

ghci> :set -XPolyKinds
ghci> data T a (b :: l) = MkT
ghci> :t MkT
MkT :: forall k l (a :: k) (b :: l). T a b
ghci> :set -fprint-explicit-kinds
ghci> :t MkT
MkT :: forall k l (a :: k) (b :: l). T @{k} @l a b
ghci> :set -XNoPolyKinds
ghci> :t MkT
MkT :: T @{*} @* a b

In the output above, observe that T has two kind variables (k and l) and two type variables (a and b). Note that k is an inferred variable and l is a specified variable (see Inferred vs. specified type variables), so as a result, they are displayed using slightly different syntax in the type T @{k} @l a b. The application of l (with @l) is the standard syntax for visible type application (see Visible type application). The application of k (with @{k}), however, uses a hypothetical syntax for visible type application of inferred type variables. This syntax is not currently exposed to the programmer, but it is nevertheless displayed when -fprint-explicit-kinds is enabled.

-fprint-explicit-coercions¶: Using -fprint-explicit-coercions makes GHC print coercions in types. When trying to prove the equality between types of different kinds, GHC uses type-level coercions. Users will rarely need to see these, as they are meant to be internal.

-fprint-axiom-incomps¶

Using -fprint-axiom-incomps tells GHC to display incompatibilities between closed type families’ equations, whenever they are printed by :info or --show-iface ⟨file⟩.

ghci> :i Data.Type.Equality.==
type family (==) (a :: k) (b :: k) :: Bool
  where
      (==) (f a) (g b) = (f == g) && (a == b)
      (==) a a = 'True
      (==) _1 _2 = 'False
ghci> :set -fprint-axiom-incomps
ghci> :i Data.Type.Equality.==
type family (==) (a :: k) (b :: k) :: Bool
  where
      {- #0 -} (==) (f a) (g b) = (f == g) && (a == b)
      {- #1 -} (==) a a = 'True
          -- incompatible with: #0
      {- #2 -} (==) _1 _2 = 'False
          -- incompatible with: #1, #0

The equations are numbered starting from 0, and the comment after each equation refers to all preceding equations it is incompatible with.

-fprint-equality-relations¶: Using -fprint-equality-relations tells GHC to distinguish between its equality relations when printing. For example, ~ is homogeneous lifted equality (the kinds of its arguments are the same) while ~~ is heterogeneous lifted equality (the kinds of its arguments might be different) and ~# is heterogeneous unlifted equality, the internal equality relation used in GHC’s solver. Generally, users should not need to worry about the subtleties here; ~ is probably what you want. Without -fprint-equality-relations, GHC prints all of these as ~. See also Equality constraints.

-fprint-expanded-synonyms¶

When enabled, GHC also prints type-synonym-expanded types in type errors. For example, with this type synonyms:

type Foo = Int
type Bar = Bool
type MyBarST s = ST s Bar

This error message:

Couldn't match type 'Int' with 'Bool'
Expected type: ST s Foo
  Actual type: MyBarST s

Becomes this:

Couldn't match type 'Int' with 'Bool'
Expected type: ST s Foo
  Actual type: MyBarST s
Type synonyms expanded:
Expected type: ST s Int
  Actual type: ST s Bool

-fprint-redundant-promotion-ticks¶

The DataKinds extension allows us to use data constructors at the type level:

type B = True     -- refers to the data constructor True (of type Bool)

When there is a type constructor of the same name, it takes precedence during name resolution:

data True = MkT
type B = True     -- now refers to the type constructor (of kind Type)

We can tell GHC to prefer the data constructor over the type constructor using special namespace disambiguation syntax that we call a promotion tick:

data True = MkT
type B = 'True
    -- refers to the data constructor True (of type Bool)
    -- even in the presence of a type constructor of the same name

Note that the promotion tick is not a promotion operator. Its only purpose is to instruct GHC to prefer the promoted data constructor over a type constructor in case of a name conflict. Therefore, GHC will not print the tick when the name conflict is absent:

ghci> type B = False
ghci> :kind! B
B :: Bool
= False          -- no promotion tick here

ghci> data False -- introduce a name conflict

ghci> :kind! B
B :: Bool
= 'False         -- promotion tick resolves the name conflict

The -fprint-redundant-promotion-ticks instructs GHC to print the promotion tick unconditionally.

-fprint-typechecker-elaboration¶

When enabled, GHC also prints extra information from the typechecker in warnings. For example:

main :: IO ()
main = do
  return $ let a = "hello" in a
  return ()

This warning message:

A do-notation statement discarded a result of type ‘[Char]’
Suppress this warning by saying
  ‘_ <- ($) return let a = "hello" in a’
or by using the flag -fno-warn-unused-do-bind

Becomes this:

A do-notation statement discarded a result of type ‘[Char]’
Suppress this warning by saying
  ‘_ <- ($)
          return
          let
            AbsBinds [] []
              {Exports: [a <= a
                           <>]
               Exported types: a :: [Char]
                               [LclId, Str=DmdType]
               Binds: a = "hello"}
          in a’
or by using the flag -fno-warn-unused-do-bind

-fdefer-diagnostics¶: Causes GHC to group diagnostic messages by severity and output them after other messages when building a multi-module Haskell program. This flag can make diagnostic messages more visible when used in conjunction with --make and -j[⟨n⟩]. Otherwise, it can be hard to find the relevant errors or likely to ignore the warnings when they are mixed with many other messages.

-fdiagnostics-color=⟨always|auto|never⟩¶

Causes GHC to display error messages with colors. To do this, the terminal must have support for ANSI color codes, or else garbled text will appear. The default value is auto, which means GHC will make an attempt to detect whether terminal supports colors and choose accordingly.

The precise color scheme is controlled by the environment variable GHC_COLORS (or GHC_COLOURS). This can be set to colon-separated list of key=value pairs. These are the default settings:

header=:message=1:warning=1;35:error=1;31:fatal=1;31:margin=1;34

Each value is expected to be a Select Graphic Rendition (SGR) substring. The formatting of each element can inherit from parent elements. For example, if header is left empty, it will inherit the formatting of message. Alternatively if header is set to 1 (bold), it will be bolded but still inherits the color of message.

Currently, in the primary message, the following inheritance tree is in place:

message
- header
  - warning
  - error
  - fatal

In the caret diagnostics, there is currently no inheritance at all between margin, warning, error, and fatal.

The environment variable can also be set to the magical values never or always, which is equivalent to setting the corresponding -fdiagnostics-color flag but with lower precedence.

-fdiagnostics-show-caret¶

Default:	on

Controls whether GHC displays a line of the original source code where the error was detected. This also affects the associated caret symbol that points at the region of code at fault.

-fshow-error-context¶

Default:	on

Controls whether GHC displays information about the context in which an error occurred. This controls whether the part of the error message which says “In the equation..”, “In the pattern..” etc is displayed or not.

-ferror-spans¶

Causes GHC to emit the full source span of the syntactic entity relating to an error message. Normally, GHC emits the source location of the start of the syntactic entity only.

For example:

test.hs:3:6: parse error on input `where'

becomes:

test296.hs:3:6-10: parse error on input `where'

And multi-line spans are possible too:

test.hs:(5,4)-(6,7):
    Conflicting definitions for `a'
    Bound at: test.hs:5:4
              test.hs:6:7
    In the binding group for: a, b, a

Note that line numbers start counting at one, but column numbers start at zero. This choice was made to follow existing convention (i.e. this is how Emacs does it).

-fkeep-going¶

Since:	8.10.1

Causes GHC to continue the compilation if a module has an error. Any reverse dependencies are pruned immediately and the whole compilation is still flagged as an error. This option has no effect if parallel compilation (-j[⟨n⟩]) is in use.

-freverse-errors¶: Causes GHC to output errors in reverse line-number order, so that the errors and warnings that originate later in the file are displayed first.

-Rghc-timing¶: Prints a one-line summary of timing statistics for the GHC run. This option is equivalent to +RTS -tstderr, see RTS options to control the garbage collector.