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13 Interactive Haskell

REPL (read–eval–print loop) is provided both via Comint (inferior-haskell-mode) and an adhoc way called haskell-interactive-mode. The Comint based inferior-haskell-mode is just the REPL, it comes with the standard key bindings(like ielm or eshell).

haskell-interactive-mode comes with a different set of features:

With haskell-interactive-mode, each Haskell source buffer is associated with at most one GHCi session, so when you call haskell-process-load-file for a Haskell source buffer which has no session associated yet, you’re asked which GHCi session to create or associate with.

13.1 Goto Error

In a Haskell source buffer associated with a GHCi session, errors that prevent the file from loading are highlighted with haskell-error-face. You can move between these error lines with


is bound to haskell-goto-next-error


is bound to haskell-goto-prev-error

C-c M-p

is bound to haskell-goto-first-error

13.2 Using GHCi 8+ or GHCi-ng

If you use either of the above, then you can use these functions:

(define-key interactive-haskell-mode-map (kbd "M-.") 'haskell-mode-goto-loc)
(define-key interactive-haskell-mode-map (kbd "C-c C-t") 'haskell-mode-show-type-at)

You have to load the module before it works, after that it will remember for the current GHCi session.

13.3 Customizing

What kind of Haskell REPL haskell-interactive-mode will start up depends on the value of haskell-process-type. This can be one of the symbols auto, ghci, cabal-repl, cabal-new-repl, or stack-ghci. If it’s auto, the directory contents and available programs will be used to make a best guess at the process type. The actual process type will then determine which variables haskell-interactive-mode will access to determine the program to start and its arguments:

With each of these pairs, the the haskell-process-path-... variable needs to be a string specifying the program path, or a list of strings where the first element is the program path and the rest are initial arguments. The haskell-process-args-... is a list of strings specifying (further) command-line arguments.

13.4 Haskell Interactive Mode Setup

The most straight-forward way to get setup with Interactive Mode is to bind the right keybindings and set some customizations. This page contains a good base setup.

To enable the minor mode which activates keybindings associated with interactive mode, use:

(require 'haskell-interactive-mode)
(require 'haskell-process)
(add-hook 'haskell-mode-hook 'interactive-haskell-mode)

13.4.1 Customizations

This enables some handy and benign features.

  '(haskell-process-suggest-remove-import-lines t)
  '(haskell-process-auto-import-loaded-modules t)
  '(haskell-process-log t))

13.4.2 Haskell-mode bindings

This gives the basic ways to start a session. In a Haskell buffer:

(define-key haskell-mode-map (kbd "C-c C-l") 'haskell-process-load-or-reload)
(define-key haskell-mode-map (kbd "C-`") 'haskell-interactive-bring)
(define-key haskell-mode-map (kbd "C-c C-t") 'haskell-process-do-type)
(define-key haskell-mode-map (kbd "C-c C-i") 'haskell-process-do-info)
(define-key haskell-mode-map (kbd "C-c C-c") 'haskell-process-cabal-build)
(define-key haskell-mode-map (kbd "C-c C-k") 'haskell-interactive-mode-clear)
(define-key haskell-mode-map (kbd "C-c c") 'haskell-process-cabal)

13.4.3 Cabal-mode bindings

The below commands pretty much match the ones above, but are handy to have in cabal-mode, too:

(define-key haskell-cabal-mode-map (kbd "C-`") 'haskell-interactive-bring)
(define-key haskell-cabal-mode-map (kbd "C-c C-k") 'haskell-interactive-mode-clear)
(define-key haskell-cabal-mode-map (kbd "C-c C-c") 'haskell-process-cabal-build)
(define-key haskell-cabal-mode-map (kbd "C-c c") 'haskell-process-cabal)

13.4.4 GHCi process type

By default haskell-process-type is set to auto. It is smart enough to pick the right type based on your project structure and installed tools, but in case something goes funky or you want to explicitly set the process type and ignore the inferred type, you can customize this setting by running M-x customize-variable RET haskell-process-type RET, or by setting the code:

  '(haskell-process-type 'cabal-repl))

Here is a list of available process types:

Please, check the documentation for haskell-process-type to see how the real type is guessed, when it’s set to auto.

13.4.5 Troubleshooting

Launching your GHCi process can fail when you’re first getting setup, depending on the type you choose. If it does fail to launch, switch to the buffer *haskell-process-log* and see what’s up. The buffer contains a log of incoming/outgoing messages to the GHCi process.

13.5 Haskell Interactive Mode Tags Using GHCi

You can bind the following to use GHCi to find definitions of things:

(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-jump-to-def)

The one problem with this approach is that if your code doesn’t compile, GHCi doesn’t give any location info. So you need to make sure your code compiles and the modules you want to jump to are loaded byte-compiled.

Note: I think that when you restart GHCi you lose location information, even if you have the .o and .hi files lying around. I’m not sure. But sometimes :i foo will give foo is defined in Bar rather than foo is defined in /foo/Bar.hs:123:23.

Alternatively, you can use tags generation, which doesn’t require a valid compile.

13.5.1 Tags Setup

Make sure to install hasktags.

    $ cabal install hasktags

Then add the customization variable to enable tags generation on save:

  '(haskell-tags-on-save t))

And make sure hasktags is in your $PATH which Emacs can see.

13.5.2 Generating tags

Now, every time you run save-buffer (C-x C-s), there is a hook that will run and generate Emacs See (emacs)Tags, for the whole project directory. The resulting file will be called TAGS.

WARNING: You should be careful that your project root isn’t your home directory or something, otherwise it will traverse all the way down and take an impossibly long time.

13.5.3 Jumping to tags

Bind the following keybinding:

(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-tag-find)

To jump to the location of the top-level identifier at point, run M-x haskell-mode-tag-find or M-..

13.5.4 Hybrid: GHCi and fallback to tags

To use GHCi first and then if that fails to fallback to tags for jumping, use:

(define-key haskell-mode-map (kbd "M-.") 'haskell-mode-jump-to-def-or-tag)

13.5.5 Troubleshooting tags

Sometimes a TAGS file is deleted (by you or some other process). Emacs will complain that it doesn’t exist anymore. To resolve this simply do M-x tags-reset-tags-tables.

13.6 Sessions

All commands in Haskell Interactive Mode work within a session. Consider it like a “project” or a “solution” in popular IDEs. It tracks the root of your project and an associated process and REPL.

13.6.1 Start a session

To start a session run the following steps:

It will prompt for a Cabal directory and a current directory. It figures out where the cabal directory is and defaults for the current directory, so you should be able to just hit RET twice.

13.6.2 Switch a session

Sometimes a particular file is used in two different sessions/projects. You can run

    M-x haskell-session-change

If it prompts you to make a new session, tell it no (that’s a bug). It will ask you to choose from a list of sessions.

13.6.3 Killing a session

To kill a session you can run

    M-x haskell-session-kill

Which will prompt to kill all associated buffers, too. Hit ‘n‘ to retain them.

Alternatively, you can switch to the REPL and just kill the buffer normally with C-x k RET. It will prompt

    Kill the whole session (y or n)?

You can choose y to kill the session itself, or n to just kill the REPL buffer. You can bring it back with M-x haskell-interactive-bring.

13.6.4 Menu

To see a list of all sessions you have open with some simple statistics about memory usage, etc. run

    M-x haskell-menu

For example:

    foo  14648 08:21:42 214MB /path/to/fpco/foo/  /path/to/fpco/foo/ ghci
    bar  29119 00:22:03 130MB /path/to/bar/       /path/to/bar/      ghci
    mu   22575 08:48:20 73MB  /path/to/fpco/mu/   /path/to/fpco/mu/  ghci

13.7 Compiling

There are a bunch of ways to compile Haskell modules. This page covers a few of them.

13.7.1 Load into GHCi

To compile and load a Haskell module into GHCi, run the following

    M-x haskell-process-load

Or C-c C-l. You’ll see any compile errors in the REPL window.

13.7.2 Build the Cabal project

To compile the whole Cabal project, run the following

    M-x haskell-process-cabal-build

Or C-c C-c. You’ll see any compile errors in the REPL window.

13.7.3 Reloading modules

To reload the current module, even when you’re in other modules, you can run C-u M-x haskell-process-load-or-reload or C-u C-c C-l. It will now reload that module whenever you run C-c C-l in the future from whatever module you’re in. To disable this mode, just run C-u C-c C-l again.

13.7.4 Jumping to compile errors

You can use the standard compile error navigation function C-x ` — jump to the next error.

Or you can move your cursor to an error in the REPL and hit RET to jump to it.

13.7.5 Auto-removing imports

If the customization variable haskell-process-suggest-remove-import-lines is enabled.

  '(haskell-process-suggest-remove-import-lines t))

Building and loading modules which output warnings like,

    Warning: The import of `Control.Monad' is redundant
      except perhaps to import instances from `Control.Monad'
    To import instances alone, use: import Control.Monad()

will prompt the user with

> The import line `Control.Monad' is redundant. Remove? (y, n, c: comment out)

If you answer

13.7.6 Auto-adding of modules to import

Enable the customization variable haskell-process-suggest-hoogle-imports.

  '(haskell-process-suggest-hoogle-imports t))

Whenever GHC says something is not in scope, it will hoogle that symbol. If there are results, it will prompt to add one of the modules from Hoogle’s results.

You need to make sure you’ve generated your Hoogle database properly.

13.7.7 Auto-adding of extensions

It you use an extension which is not enabled, GHC will often inform you. For example, if you write:

newtype X a = X (IO a)
  deriving (Monad)

Then you’ll see a message like:

    x.hs:13:13: Can't make a derived instance of `Monad X': …
          `Monad' is not a derivable class
          Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension
        In the newtype declaration for `X'

This -XFoo pattern will be picked up and you will be prompted:

> Add `{-# LANGUAGE GeneralizedNewtypeDeriving #-}` to the top of the
> file? (y or n)

If you answer ‘y‘, it will temporarily jump to the buffer and it to the top of the file.

13.7.8 Orphan instances

If GHC complains about orphan instances, you usually are doing it intentionally, so it prompts to add -fno-warn-orphans to the top of the file with an OPTIONS pragma.

13.7.9 Auto-adding of dependencies

When doing a build, you will sometimes get a message from GHC like:

    src/ACE/Tokenizer.hs:11:18: Could not find module `Data.Attoparsec.Text' …
        It is a member of the hidden package `attoparsec-'.

This message contains all the necessary information to add this to your .cabal file, so you will be prompted to add it to your .cabal file:

    Add `attoparsec' to ace.cabal? (y or n)  y

If you hit y, it will prompt with this:

    attoparsec >=

Which you can edit (e.g. do some PVP decision or remove constraints entirely), and then it will open up your .cabal file and go through each section:

    Add to library? (y or n)  y

This will add it to the top of the build-depends field in your library section. If you have any executables, it will go through each of those, prompting, too.

Now you can rebuild with C-c C-c again.

13.8 Haskell Interactive Mode REPL

When GHCi has been launched, it works on a read-eval-print basis. So you will be presented with the prompt:

    The lambdas must flow.
    Changed directory: /path/to/your/project/

13.8.1 Changing REPL target

With haskell-session-change-target you can change the target for REPL session.

After REPL session started, in haskell-interactive-mode buffer invoke the haskell-session-change-target and select from available targets for

- Testing

- Benchmark

- Executable

- Library

Answer “yes” to restart the session and run your tests, benchmarks, executables.


13.8.2 Bringing the REPL

If you don’t know where the REPL buffer is, you can always bring it with:

    M-x haskell-interactive-bring

Or C-`.

13.8.3 Evaluating expressions

To evaluate expressions, simply type one out and hit ‘RET‘.

    λ> 123

13.8.4 Evaluating multiline expressions

GHCi features two ways to evaluate multiline expressions. You can use :set +m to enable multiline input for all expressions, or you can wrap your expression in :{ and :} (they have to be on their own lines).

The prompt will change to indicate that you’re inputting a multiline expression:

λ> :{
λ| let a = 10
λ|     b = 20
λ|     c = 30
λ| :}

You can also simulate multiline mode by having your input contain newline characters. You can input a literal newline character with C-q C-j, or you can use:

    M-x haskell-interactive-mode-newline-indent

which is bound to C-j. This command indents after the newline. You can simulate the above example like so:

λ> let a = 10
       b = 20
       c = 30

13.8.5 Type of expressions

You can use normal :type which is part of GHCi to get the type of something:

    λ> :t id
    id :: a -> a

But you can also just write out the value directly,

    λ> id
    id :: a -> a

and because there’s no Show instance for (a -> a). This would normally yield a compile error:

    No instance for (Show (a0 -> a0))
      arising from a use of `print'
    Possible fix: add an instance declaration for (Show (a0 -> a0))
    In a stmt of an interactive GHCi command: print it

It will run :type id in the background and print out the result. The same is true for ambiguous things:

    λ> :t read "a"
    read "a" :: Read a => a

Because this would normally be an ambiguous constraint:

    Ambiguous type variable `a0' in the constraint:
      (Read a0) arising from a use of `read'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: read \"a\"
    In an equation for `it': it = read \"a\"

Which is less useful than just printing the type out.

You can disable this behaviour by disabling the customization option:

  '(haskell-interactive-types-for-show-ambiguous nil))

13.8.6 Printing mode

You can choose between printing modes used for the results of evaluating expressions. To do that, configure the variable haskell-interactive-mode-eval-mode. Example:

(setq haskell-interactive-mode-eval-mode 'haskell-mode)

A handy function you can use is:

(defun haskell-interactive-toggle-print-mode ()
  (setq haskell-interactive-mode-eval-mode
         (ido-completing-read "Eval result mode: "

(Add whichever modes you want to use.)

And then run

    M-x haskell-interactive-toggle-print-mode

Or C-c C-v:

(define-key haskell-interactive-mode-map (kbd "C-c C-v")

There you can choose ‘haskell-mode‘, for example, to pretty print the output as Haskell.

13.8.7 SVG images rendering

If you are working on SVG images, you can instruct Emacs to render the image as the output of an image producing command at the REPL.

The following example uses the diamgrams library with the default SVG backend to produce a circle:

    {-# LANGUAGE OverloadedStrings #-}

    import Diagrams.Prelude
    import Diagrams.Backend.SVG

    myCircle :: Diagram B
    myCircle = circle 1 # lc purple # fc yellow

    circle = renderDia SVG (SVGOptions (mkWidth 250) Nothing "" [] True) myCircle

After enabling SVG rendering with M-x haskell-svg-toggle-render-images, if you load the above code and type circle at the REPL, you will see the rendered circle instead of the XML representation of the image.

This feature can be enabled by default by setting the customization variable haskell-svg-render-images to a non-nil value.

13.8.8 Presentations

If you have the present package installed, you can use the following syntax to print anything which is an instance of Data:

    λ> :present 123

It will print data structures lazily:

    λ> :present [1..]

It shows types when there is an unevaluated field in a constructor. You can click the [Integer] or press RET on it to expand further:

    λ> :present [1..]

Etc. Remember: this only works for instances of Data.Data.Data.

13.8.9 History

A history is maintained for the duration of the REPL buffer. To go up and down in the history, run M-p for previous and M-n for next.

13.8.10 Cancelling commands

To cancel a running REPL command, run C-c C-c.

13.8.11 Clear the REPL

Run C-c C-k to clear the REPL.

13.8.12 Trick: Put Interactive REPL in Separate Frame

The following create-haskell-interactive-frame is a quick hack to move the repl to a separate frame, for those that want a more predictable layout of windows in Emacs.

(defun create-unfocused-frame ()
    ((prv (window-frame))
     (created (make-frame)))
    (select-frame-set-input-focus prv) created))

(defun create-haskell-interactive-frame ()

13.8.13 Troubleshooting

If the REPL ever goes funny, you can clear the command queue via:

    M-x haskell-process-clear

Alternatively, you can just restart the process:

    M-x haskell-process-restart

You can also switch to the buffer *haskell-process-log*, which can be enabled and disabled with the customization variable ‘haskell-process-log‘, to see what the cause of your troubles are.

If the process fails and nothing unusual is in the process log, the following command can dump the haskell-process state:

    M-: (haskell-process)

The output can be copied from the *Messages* buffer.

13.9 Haskell Interactive Mode Querying

There a few ways GHCi lets you query information about your code.

13.9.1 Get identifer type

To print the type of the top-level identifier at point in the REPL and in the message buffer, run the following command:

    M-x haskell-process-do-type

or C-c C-t.

13.9.2 Insert identifier’s type as type signature

To print the type of the top-level identifier at point, run the following command:

    C-u M-x haskell-process-do-type

or C-u C-c C-t.

13.9.3 Get identifier info

To print the info of the identifier at point, run the following command:

    M-x haskell-process-do-info

or C-c C-i.

13.9.4 Presentation mode

When using C-c C-i or C-c C-t it will open a buffer in haskell-presentation-mode. You can hit q to close the buffer.

But you can also continue to use C-c C-i inside the buffer to drill further down data types and classes.

E.g. if you go to Ord in your code buffer and C-c C-i, it will popup a buffer containing

class Eq a => Ord a where
  compare :: a -> a -> Ordering
  (<) :: a -> a -> Bool
  (>=) :: a -> a -> Bool
  (>) :: a -> a -> Bool
  (<=) :: a -> a -> Bool
  max :: a -> a -> a
  min :: a -> a -> a
  	-- Defined in `GHC.Classes'

And all the instances of that class. But then you can also move your cursor to Ordering and hit C-c C-i again to get another popup:

data Ordering = LT | EQ | GT 	-- Defined in `GHC.Types'
instance Bounded Ordering -- Defined in `GHC.Enum'
instance Enum Ordering -- Defined in `GHC.Enum'
instance Eq Ordering -- Defined in `GHC.Classes'
instance Ord Ordering -- Defined in `GHC.Classes'
instance Read Ordering -- Defined in `GHC.Read'
instance Show Ordering -- Defined in `GHC.Show'

And so on. It’s a very good way of exploring a new codebase.

13.9.5 Browse import’s module

To print all exported identifiers of the module imported by the import line at point, run the following command:

    M-x haskell-process-do-info

or C-c C-i. It will print all exports by running :browse The.Module in the GHCi process.

13.10 Haskell Interactive Mode Cabal integration

There’s some integration with Cabal in Haskell Interactive Mode. Once you’ve started a session, the features below are available.

13.10.1 Cabal building

The most common Cabal action is building, so that has a specific command:

    M-x haskell-process-cabal-build

Or C-c C-c. When building, it will hide unneccessary output.

For example, to build the ‘ace‘ package, the output is simply:

    Compiling: ACE.Types.Tokens
    Compiling: ACE.Combinators
    Compiling: ACE.Tokenizer
    Compiling: ACE.Parsers
    Compiling: ACE.Pretty
    Compiling: ACE
    Complete: cabal build (0 compiler messages)

Whereas the complete output is normally:

    Building ace-0.5...
    Preprocessing library ace-0.5...
    [4 of 9] Compiling ACE.Types.Tokens ( src/ACE/Types/Tokens.hs, dist/build/ACE/Types/Tokens.o )
    [5 of 9] Compiling ACE.Combinators  ( src/ACE/Combinators.hs, dist/build/ACE/Combinators.o ) [ACE.Types.Tokens changed]
    [6 of 9] Compiling ACE.Tokenizer    ( src/ACE/Tokenizer.hs, dist/build/ACE/Tokenizer.o ) [ACE.Types.Tokens changed]
    [7 of 9] Compiling ACE.Parsers      ( src/ACE/Parsers.hs, dist/build/ACE/Parsers.o )
    [8 of 9] Compiling ACE.Pretty       ( src/ACE/Pretty.hs, dist/build/ACE/Pretty.o )
    [9 of 9] Compiling ACE              ( src/ACE.hs, dist/build/ACE.o ) [ACE.Tokenizer changed]
    In-place registering ace-0.5...

Which is considerably more verbose but rarely useful or interesting.

13.10.2 Arbitrary cabal commands

To run an arbitrary Cabal command:

    C-u M-x haskell-process-cabal

Or run C-u C-c c.

It will prompt for an input, so you can write configure -fdev, for example.

13.10.3 Completing cabal commands

To run some common Cabal commands, just run:

    M-x haskell-process-cabal

Or C-c c. This is commonly used to do install, haddock, configure, etc.

13.11 Haskell Interactive Mode Debugger

There is limited support for debugging in GHCi. Haskell Interactive Mode provides an interface for interacting with this.

13.11.1 Opening the debug buffer

To open the debug buffer run the following command from any buffer associated with a session:

    M-x haskell-debug

It will open a buffer that looks like this:

    Debugging haskell

    You have to load a module to start debugging.

    g - refresh


    No loaded modules.

13.11.2 Loading modules

To debug anything you need to load something into GHCi. Switch to a normal file, for example:

main = do putStrLn "Hello!"
          putStrLn "World"

and load it into GHCi (C-c C-l). Now when you hit g (to refresh) in the debugging buffer, you’ll see something like:

    Debugging haskell

    b - breakpoint, g - refresh


    Not debugging right now.


    No active breakpoints.


    Main - hello.hs

13.11.3 Setting a breakpoint

To set a breakpoint hit b in the debugger buffer. It will prompt for a name. Enter main and hit RET.

Now the buffer will look like this:

    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, g - refresh


    Not debugging right now.


    0 - Main (1:8)


    Main - hello.hs

13.11.4 Start stepping

Hit s to step through an expression: it will prompt for an expression to evaluate and step through. Enter main and hit RET. Now the buffer will look like this:

    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, a - abandon context, c - continue
    p - previous step, n - next step
    g - refresh


    main - hello.hs (stopped)

    do putStrLn "Hello!"
       putStrLn "World"

    _result :: IO () = _

       1 do putStrLn "Hello!" putStrLn "World"


    0 - Main (1:8)


    Main - hello.hs

What we see here is the current expression being evaluated:

do putStrLn "Hello!"
   putStrLn "World"

And we see the type of it:

_result :: IO () = _

And we see a backtrace of steps so far:

1 do putStrLn "Hello!" putStrLn "World"

13.11.5 Continue stepping

To continue stepping, just hit s again. Now the context will change to:

main - hello.hs (stopped)

putStrLn "Hello!"

_result :: IO () = _

   1 do putStrLn "Hello!" putStrLn "World"

Hitting s once more, we see the context change to:

putStrLn "World"

_result :: IO () = _

   2 putStrLn "Hello!"
   1 do putStrLn "Hello!" putStrLn "World"

Finally hitting s again will say "Computation finished". Hitting s a final time will change the display back to:

    Debugging haskell

    s - step into an expression, b - breakpoint
    d - delete breakpoint, g - refresh


    Finished debugging.

       2 putStrLn "Hello!"
       1 do putStrLn "Hello!" putStrLn "World"


    1 - Main (1:8)


    Main - hello.hs

And you’re done debugging.

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