Haskell: :set : Change settings

By using ‘:set’ command we can change some of the default behaviors of the Haskell Environment.

Following are the commands to change settings.

Command	Description
:set <option> ...	Set options
:seti <option> ...	Set options for interactive evaluation only
:set args <arg> ...	Set the arguments returned by System.getArgs
:set prog <progname>	Set the value returned by System.getProgName
:set prompt <prompt>	Set the prompt used in GHCi
:set prompt2 <prompt>	Set the continuation prompt used in GHCi
:set editor <cmd>	Set the command used for :edit

Following are the various options used by both ‘:set’ ‘:unset’ commands.

Option	Description
+m	Allow multiline commands
+r	Revert top-level expressions after each evaluation
+s	Print timing/memory stats after each evaluation
+t	Print type after evaluation
-<flags>	Most GHC command line flags can also be set here. (eg. -v2, -XFlexibleInstances, etc.)

Change the GHCi prompt
By using ‘:set prompt promptName’ command, you can change the prompt name used in GHCi.

Prelude> :set prompt ptr>
ptr>
ptr>let str1 = "Hello World"
ptr>putStrLn str1
Hello World
ptr>

By using various options of :set command, you can add extra information while displaying variables.

ptr>let str1 = "Hello World"
ptr>str1
"Hello World"
ptr>
ptr>:set +t +s
ptr>
ptr>str1
"Hello World"
it :: [Char]
(0.00 secs, 514,752 bytes)
ptr>

‘:set +t +s’ add type information of the variable,  timing/memory stats of variable after every evaluation.

References

https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci-set.html

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