stack ghci and
stack repl commands¶
stack ghci [TARGET/FILE] [--pedantic] [--ghci-options OPTIONS] [--ghc-options OPTIONS] [--flag PACKAGE:[-]FLAG] [--with-ghc GHC] [--[no-]load] [--package PACKAGE] [--main-is TARGET] [--load-local-deps] [--[no-]package-hiding] [--only-main] [--trace] [--profile] [--no-strip] [--[no-]test] [--[no-]bench]
A read–evaluate–print loop (REPL) environment takes single user inputs, executes
them, and returns the result to the user. GHCi is GHC's interactive environment.
stack ghci or
stack repl commands, which are equivalent, allow you to
load components and files of your project into GHCi.
The command uses the same TARGET syntax as
stack build. It can also take flags
--bench and options like
--flag. Similarly to
stack build, the default is to load up GHCi with all libraries and executables
in the project.
In order to load multiple components,
stack ghci combines all of the GHC
options together. This doesn't work in the general case, but when the packages
being loaded share similar conventions, it should work out. A common source of
issues is when one component defines default extensions which aren't assumed by
another component. For example, specifying
NoImplicitPrelude in one component
but not another is quite likely to cause failures. GHCi will be run with
-XNoImplicitPrelude, but it is likely that modules in the other component
assume that the
Prelude is implicitly imported.
Selecting Main module¶
When loading multiple packages, there may be multiple definitions for the
module. You can specify which
Main module to load with the
--main-is <target> option. If no selection is made and there are multiple
Main modules, you will be asked to select from a list of options.
Speeding up initial load¶
There are two ways to speed up the initial startup of GHCi:
--no-buildflag, to skip an initial build step. This works only if the dependencies have already been built.
--no-loadflag, to skip loading all defined modules into GHCi. You can then directly use
:load MyModulein GHCi to load a specific module in your project.
Loading just the main module¶
stack ghci loads and imports all of the modules in the package.
This allows you to easily use anything exported by your package. This is usually
quite convenient, but in some cases it makes sense to only load one module, or
no modules at all. The
--only-main flag allows this. It specifies that only
the main module will be loaded, if any. This is particularly useful in the
You're loading the project in order to run it in GHCi (e.g. via
main), and you intend to reload while developing. Without the
--only-mainflag, you will need to quit and restart GHCi whenever a module gets deleted. With the flag, reloading should work fine in this case.
If many of your modules have exports named the same thing, then you'll need to refer to them using qualified names. To avoid this, it may be easier to use the
--only-mainflag to start with a blank slate and just import the modules you are interested in.
Loading a filepath directly¶
Instead of the
TARGET syntax, it is also possible to command directly, for
This will figure out which component the file is associated with, and use the options from that component.
Specifying extra packages to build or depend on¶
Sometimes you want to load GHCi with an additional package, that isn't a direct
dependency of your components. This can be achieved by using the
option. For example, if I want to experiment with the
lens library, I can
Running plain GHCi¶
stack ghci always runs GHCi configured to load code from packages in your
project. In particular, this means it passes in flags like
-package-id= in order to configure which packages are visible to GHCi.
For doing experiments which just involve packages installed in your databases, it may be useful to run GHCi plainly like:
This will run a plain GHCi in an environment which includes
and so will have access to your databases.