2.7. GHCi commands
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## 2.7. GHCi commands

GHCi commands all begin with ‘`:`’ and consist of a single command name followed by zero or more parameters. The command name may be abbreviated, with ambiguities being resolved in favour of the more commonly used commands.

`:abandon`

  Abandons the current evaluation (only available when stopped at a breakpoint).

`:add` [`*`]`module` ...

  Add `module`(s) to the current target set, and perform a reload. Normally pre-compiled code for the module will be loaded if available, or otherwise the module will be compiled to byte-code. Using the `*` prefix forces the module to be loaded as byte-code.

`:back`

  Travel back one step in the history. See Section 2.5.5, “Tracing and history”. See also: `:trace`, `:history`, `:forward`.

`:break [`identifier` | [`module`] `line` [`column`]]`

  Set a breakpoint on the specified function or line and column. See Section 2.5.1.1, “Setting breakpoints”.

`:browse`[`!`] [[`*`]`module`] ...

  Displays the identifiers defined by the module `module`, which must be either loaded into GHCi or be a member of a package. If `module` is omitted, the most recently-loaded module is used.

  If the `*` symbol is placed before the module name, then all the identifiers in scope in `module` are shown; otherwise the list is limited to the exports of `module`. The `*`-form is only available for modules which are interpreted; for compiled modules (including modules from packages) only the non-`*` form of `:browse` is available. If the `!` symbol is appended to the command, data constructors and class methods will be listed individually, otherwise, they will only be listed in the context of their data type or class declaration. The `!`-form also annotates the listing with comments giving possible imports for each group of entries.

      Prelude> :browse! Data.Maybe
      -- not currently imported
      Data.Maybe.catMaybes :: [Maybe a] -> [a]
      Data.Maybe.fromJust :: Maybe a -> a
      Data.Maybe.fromMaybe :: a -> Maybe a -> a
      Data.Maybe.isJust :: Maybe a -> Bool
      Data.Maybe.isNothing :: Maybe a -> Bool
      Data.Maybe.listToMaybe :: [a] -> Maybe a
      Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
      Data.Maybe.maybeToList :: Maybe a -> [a]
      -- imported via Prelude
      Just :: a -> Maybe a
      data Maybe a = Nothing | Just a
      Nothing :: Maybe a
      maybe :: b -> (a -> b) -> Maybe a -> b

  This output shows that, in the context of the current session, in the scope of `Prelude`, the first group of items from `Data.Maybe` have not been imported (but are available in fully qualified form in the GHCi session - see Section 2.4.3, “What's really in scope at the prompt?”), whereas the second group of items have been imported via `Prelude` and are therefore available either unqualified, or with a `Prelude.` qualifier.

`:cd` `dir`

  Changes the current working directory to `dir`. A ‘`˜`’ symbol at the beginning of `dir` will be replaced by the contents of the environment variable `HOME`.

  NOTE: changing directories causes all currently loaded modules to be unloaded. This is because the search path is usually expressed using relative directories, and changing the search path in the middle of a session is not supported.

`:cmd` `expr`

  Executes `expr` as a computation of type `IO String`, and then executes the resulting string as a list of GHCi commands. Multiple commands are separated by newlines. The `:cmd` command is useful with `:def` and `:set stop`.

`:continue`

  Continue the current evaluation, when stopped at a breakpoint.

`:ctags` [`filename`] `:etags` [`filename`]

  Generates a “tags” file for Vi-style editors (`:ctags`) or Emacs-style editors (`:etags`). If no filename is specified, the default `tags` or `TAGS` is used, respectively. Tags for all the functions, constructors and types in the currently loaded modules are created. All modules must be interpreted for these commands to work.

`:def[!] [`name` `expr`]`

  `:def` is used to define new commands, or macros, in GHCi. The command `:def` `name` `expr` defines a new GHCi command `:`name``, implemented by the Haskell expression `expr`, which must have type `String -> IO String`. When `:`name` `args`` is typed at the prompt, GHCi will run the expression `(`name` `args`)`, take the resulting `String`, and feed it back into GHCi as a new sequence of commands. Separate commands in the result must be separated by ‘`\n`’.

  That's all a little confusing, so here's a few examples. To start with, here's a new GHCi command which doesn't take any arguments or produce any results, it just outputs the current date & time:

      Prelude> let date _ = Time.getClockTime >>= print >> return ""
      Prelude> :def date date
      Prelude> :date
      Fri Mar 23 15:16:40 GMT 2001

  Here's an example of a command that takes an argument. It's a re-implementation of `:cd`:

      Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
      Prelude> :def mycd mycd
      Prelude> :mycd ..

  Or I could define a simple way to invoke “`ghc ––make Main`” in the current directory:

      Prelude> :def make (\_ -> return ":! ghc ––make Main")

  We can define a command that reads GHCi input from a file. This might be useful for creating a set of bindings that we want to repeatedly load into the GHCi session:

      Prelude> :def . readFile
      Prelude> :. cmds.ghci

  Notice that we named the command `:.`, by analogy with the ‘`.`’ Unix shell command that does the same thing.

  Typing `:def` on its own lists the currently-defined macros. Attempting to redefine an existing command name results in an error unless the `:def!` form is used, in which case the old command with that name is silently overwritten.

`:delete * | `num` ...`

  Delete one or more breakpoints by number (use `:show breaks` to see the number of each breakpoint). The `*` form deletes all the breakpoints.

`:edit [`file`]`

  Opens an editor to edit the file `file`, or the most recently loaded module if `file` is omitted. The editor to invoke is taken from the `EDITOR` environment variable, or a default editor on your system if `EDITOR` is not set. You can change the editor using `:set editor`.

`:etags`

  See `:ctags`.

`:force `identifier` ...`

  Prints the value of `identifier` in the same way as `:print`. Unlike `:print`, `:force` evaluates each thunk that it encounters while traversing the value. This may cause exceptions or infinite loops, or further breakpoints (which are ignored, but displayed).

`:forward`

  Move forward in the history. See Section 2.5.5, “Tracing and history”. See also: `:trace`, `:history`, `:back`.

`:help` , `:?`

  Displays a list of the available commands.

`:`

  Repeat the previous command.

`:history [`num`]`

  Display the history of evaluation steps. With a number, displays that many steps (default: 20). For use with `:trace`; see Section 2.5.5, “Tracing and history”.

`:info` `name` ...

  Displays information about the given name(s). For example, if `name` is a class, then the class methods and their types will be printed; if `name` is a type constructor, then its definition will be printed; if `name` is a function, then its type will be printed. If `name` has been loaded from a source file, then GHCi will also display the location of its definition in the source.

  For types and classes, GHCi also summarises instances that mention them. To avoid showing irrelevant information, an instance is shown only if (a) its head mentions `name`, and (b) all the other things mentioned in the instance are in scope (either qualified or otherwise) as a result of a `:load` or `:module` commands.

`:kind` `type`

  Infers and prints the kind of `type`. The latter can be an arbitrary type expression, including a partial application of a type constructor, such as `Either Int`.

`:load` [`*`]`module` ...

  Recursively loads the specified `module`s, and all the modules they depend on. Here, each `module` must be a module name or filename, but may not be the name of a module in a package.

  All previously loaded modules, except package modules, are forgotten. The new set of modules is known as the target set. Note that `:load` can be used without any arguments to unload all the currently loaded modules and bindings.

  Normally pre-compiled code for a module will be loaded if available, or otherwise the module will be compiled to byte-code. Using the `*` prefix forces a module to be loaded as byte-code.

  After a `:load` command, the current context is set to:

  - `module`, if it was loaded successfully, or

  - the most recently successfully loaded module, if any other modules were loaded as a result of the current `:load`, or

  - `Prelude` otherwise.

`:main `arg1` ... `argn``

  When a program is compiled and executed, it can use the `getArgs` function to access the command-line arguments. However, we cannot simply pass the arguments to the `main` function while we are testing in ghci, as the `main` function doesn't take its arguments directly.

  Instead, we can use the `:main` command. This runs whatever `main` is in scope, with any arguments being treated the same as command-line arguments, e.g.:

      Prelude> let main = System.Environment.getArgs >>= print
      Prelude> :main foo bar
      ["foo","bar"]

  We can also quote arguments which contains characters like spaces, and they are treated like Haskell strings, or we can just use Haskell list syntax:

      Prelude> :main foo "bar baz"
      ["foo","bar baz"]
      Prelude> :main ["foo", "bar baz"]
      ["foo","bar baz"]

  Finally, other functions can be called, either with the `-main-is` flag or the `:run` command:

      Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
      Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
      Prelude> :set -main-is foo
      Prelude> :main foo "bar baz"
      foo
      ["foo","bar baz"]
      Prelude> :run bar ["foo", "bar baz"]
      bar
      ["foo","bar baz"]

`:module [+|-] [*]`mod1` ... [*]`modn`` , `import `mod``

  Sets or modifies the current context for statements typed at the prompt. The form `import `mod`` is equivalent to `:module +`mod``. See Section 2.4.3, “What's really in scope at the prompt?” for more details.

`:print` `names` ...

  Prints a value without forcing its evaluation. `:print` may be used on values whose types are unknown or partially known, which might be the case for local variables with polymorphic types at a breakpoint. While inspecting the runtime value, `:print` attempts to reconstruct the type of the value, and will elaborate the type in GHCi's environment if possible. If any unevaluated components (thunks) are encountered, then `:print` binds a fresh variable with a name beginning with `_t` to each thunk. See Section 2.5.1, “Breakpoints and inspecting variables” for more information. See also the `:sprint` command, which works like `:print` but does not bind new variables.

`:quit`

  Quits GHCi. You can also quit by typing control-D at the prompt.

`:reload`

  Attempts to reload the current target set (see `:load`) if any of the modules in the set, or any dependent module, has changed. Note that this may entail loading new modules, or dropping modules which are no longer indirectly required by the target.

`:run`

  See `:main`.

`:set` [`option`...]

  Sets various options. See Section 2.8, “The `:set` command” for a list of available options and Section 4.19.10, “Interactive-mode options” for a list of GHCi-specific flags. The `:set` command by itself shows which options are currently set. It also lists the current dynamic flag settings, with GHCi-specific flags listed separately.

`:set` `args` `arg` ...

  Sets the list of arguments which are returned when the program calls `System.getArgs`.

`:set` `editor` `cmd`

  Sets the command used by `:edit` to `cmd`.

`:set` `prog` `prog`

  Sets the string to be returned when the program calls `System.getProgName`.

`:set` `prompt` `prompt`

  Sets the string to be used as the prompt in GHCi. Inside `prompt`, the sequence `%s` is replaced by the names of the modules currently in scope, and `%%` is replaced by `%`. If `prompt` starts with " then it is parsed as a Haskell String; otherwise it is treated as a literal string.

`:set` `stop` [`num`] `cmd`

  Set a command to be executed when a breakpoint is hit, or a new item in the history is selected. The most common use of `:set stop` is to display the source code at the current location, e.g. `:set stop :list`.

  If a number is given before the command, then the commands are run when the specified breakpoint (only) is hit. This can be quite useful: for example, `:set stop 1 :continue` effectively disables breakpoint 1, by running `:continue` whenever it is hit (although GHCi will still emit a message to say the breakpoint was hit). What's more, with cunning use of `:def` and `:cmd` you can use `:set stop` to implement conditional breakpoints:

      *Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
      *Main> :set stop 0 :cond (x < 3)

  Ignoring breakpoints for a specified number of iterations is also possible using similar techniques.

`:show bindings`

  Show the bindings made at the prompt and their types.

`:show breaks`

  List the active breakpoints.

`:show context`

  List the active evaluations that are stopped at breakpoints.

`:show modules`

  Show the list of modules currently loaded.

`:show packages`

  Show the currently active package flags, as well as the list of packages currently loaded.

`:show languages`

  Show the currently active language flags.

`:show [args|prog|prompt|editor|stop]`

  Displays the specified setting (see `:set`).

`:sprint`

  Prints a value without forcing its evaluation. `:sprint` is similar to `:print`, with the difference that unevaluated subterms are not bound to new variables, they are simply denoted by ‘_’.

`:step [`expr`]`

  Single-step from the last breakpoint. With an expression argument, begins evaluation of the expression with a single-step.

`:trace [`expr`]`

  Evaluates the given expression (or from the last breakpoint if no expression is given), and additionally logs the evaluation steps for later inspection using `:history`. See Section 2.5.5, “Tracing and history”.

`:type` `expression`

  Infers and prints the type of `expression`, including explicit forall quantifiers for polymorphic types. The monomorphism restriction is not applied to the expression during type inference.

`:undef` `name`

  Undefines the user-defined command `name` (see `:def` above).

`:unset` `option`...

  Unsets certain options. See Section 2.8, “The `:set` command” for a list of available options.

`:!` `command`...

  Executes the shell command `command`.

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