The interactive GHCi tool

Once you have installed stack, from within a stack project, you can start the interactive tool by typing

$ stack ghci
GHCi, version 9.6.4: https://www.haskell.org/ghc/  :? for help
ghci>

Within this system there are certain commands, and you can list them by typing

ghci> :help
 Commands available from the prompt:

   <statement>                 evaluate/run <statement>
   :                           repeat last command
   :{\n ..lines.. \n:}\n       multiline command
   :add [*]<module> ...        add module(s) to the current target set
   :browse[!] [[*]<mod>]       display the names defined by module <mod>
                               (!: more details; *: all top-level names)
   :cd <dir>                   change directory to <dir>
   :cmd <expr>                 run the commands returned by <expr>::IO String
   :complete <dom> [<rng>] <s> list completions for partial input string
...

Amoung important commands are :browse that lets you display the types of the functions in a module, :doc that lets you display the documentation haddoc documentation.

You can also load or reload a program using :load and/or :reload.

Hoogle

There is a command line tool called hoogle, that lets you browse haskell functions by type, for instance:

$ hoogle "Num a => [a] -> a"
h12:05 am $ hoogle "Num a => [a] -> a"
GHC.List sum :: Num a => [a] -> a
GHC.List product :: Num a => [a] -> a
GHC.OldList sum :: Num a => [a] -> a
GHC.OldList product :: Num a => [a] -> a
...

If you prefer working outside of the command line, the tool is also available at https://hoogle.haskell.org/