2.8. The :set and :seti commands

2.8. The `:set` and `:seti` commands
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## 2.8. The `:set` and `:seti` commands

The `:set` command sets two types of options: GHCi options, which begin with ‘`+`’, and “command-line” options, which begin with ‘-’.

NOTE: at the moment, the `:set` command doesn't support any kind of quoting in its arguments: quotes will not be removed and cannot be used to group words together. For example, `:set -DFOO='BAR BAZ'` will not do what you expect.

### 2.8.1. GHCi options

GHCi options may be set using `:set` and unset using `:unset`.

The available GHCi options are:

`+m`

  Enable parsing of multiline commands. A multiline command is prompted for when the current input line contains open layout contexts (see Section 2.4.3, “Multiline input”).

`+r`

  Normally, any evaluation of top-level expressions (otherwise known as CAFs or Constant Applicative Forms) in loaded modules is retained between evaluations. Turning on `+r` causes all evaluation of top-level expressions to be discarded after each evaluation (they are still retained during a single evaluation).

  This option may help if the evaluated top-level expressions are consuming large amounts of space, or if you need repeatable performance measurements.

`+s`

  Display some stats after evaluating each expression, including the elapsed time and number of bytes allocated. NOTE: the allocation figure is only accurate to the size of the storage manager's allocation area, because it is calculated at every GC. Hence, you might see values of zero if no GC has occurred.

`+t`

  Display the type of each variable bound after a statement is entered at the prompt. If the statement is a single expression, then the only variable binding will be for the variable ‘`it`’.

### 2.8.2. Setting GHC command-line options in GHCi

Normal GHC command-line options may also be set using `:set`. For example, to turn on `-fwarn-missing-signatures`, you would say:

    Prelude> :set -fwarn-missing-signatures

Any GHC command-line option that is designated as dynamic (see the table in Section 4.20, “Flag reference”), may be set using `:set`. To unset an option, you can set the reverse option:

    Prelude> :set -fno-warn-incomplete-patterns -XNoMultiParamTypeClasses

Section 4.20, “Flag reference” lists the reverse for each option where applicable.

Certain static options (`-package`, `-I`, `-i`, and `-l` in particular) will also work, but some may not take effect until the next reload.

### 2.8.3. Setting options for interactive evaluation only

GHCi actually maintains two sets of options: one set that applies when loading modules, and another set that applies for expressions and commands typed at the prompt. The `:set` command modifies both, but there is also a `:seti` command (for "set interactive") that affects only the second set.

The two sets of options can be inspected using the `:set` and `:seti` commands respectively, with no arguments. For example, in a clean GHCi session we might see something like this:

    Prelude> :seti
    base language is: Haskell2010
    with the following modifiers:
      -XNoDatatypeContexts
      -XNondecreasingIndentation
      -XExtendedDefaultRules
    GHCi-specific dynamic flag settings:
    other dynamic, non-language, flag settings:
      -fimplicit-import-qualified
    warning settings:

Note that the option `-XExtendedDefaultRules` is on, because we apply special defaulting rules to expressions typed at the prompt (see Section 2.4.7, “Type defaulting in GHCi”).

It is often useful to change the language options for expressions typed at the prompt only, without having that option apply to loaded modules too. A good example is

    :seti -XNoMonomorphismRestriction

It would be undesirable if `-XNoMonomorphismRestriction` were to apply to loaded modules too: that might cause a compilation error, but more commonly it will cause extra recompilation, because GHC will think that it needs to recompile the module because the flags have changed.

It is therefore good practice if you are setting language options in your `.ghci` file, to use `:seti` rather than `:set` unless you really do want them to apply to all modules you load in GHCi.

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