Basics
A quickstart to Eta REPL
A quickstart to Eta REPL
You can fire up a REPL from anywhere with the following command:
$ etlas repl
...
Prelude>
You'll be greeted with a nice welcome message and then a prompt.
The text before the > will list all the modules in scope. This means that you can use any exported function or type from the listed modules. By default, Prelude is in scope, just as it would be in Eta modules written in text files.
Let's try an expression!
Prelude> "Hello" ++ " " ++ "World!"
"Hello World!"
it :: (Num a) => a
The result of evaluating the expression is printed using its Show instance. Then, the type of the expression is also printed. Note that the expression is bound to an implicit binder called it which you can use later.
Prelude> it
"Hello World!"
it :: [Char]
Prelude> replicate 10 'c'
"cccccccccc"
it :: [Char]
Prelude> it
"cccccccccc"
it :: [Char]
If an expression does not have a Show instance, the REPL throws an error.
Prelude> \x -> x + 1
<interactive>:2:1: error:
No instance for (Show (a0 -> a0))
(maybe you haven't applied enough arguments to a function?)
arising from a use of 'print'
In the first argument of 'print', namely 'it'
In a stmt of an interactive GHCi command: print it
If the expression is an IO action (of type IO a), then the REPL will evaluate the IO action and execute it!
Prelude> print "hello"
"hello"
it :: ()
Note that the type of it is () and not IO () which is the type of the expression print "hello". This is because the IO action has been run and it is bound to the result of the action instead of the action itself, which is what we want most of the time.
Suppose we wanted to use the sort function defined in Data.List.
Prelude> sort [10,9..1]
<interactive>:2:1: error:
Not in scope: 'sort'
Perhaps you meant 'sqrt' (imported from Prelude)
Prelude> import Data.List
Prelude Data.List> sort [10,9..1]
[1,2,3,4,5,6,7,8,9,10]
it :: (Enum a, Num a, Ord a) => [a]
You can use the import statement to import modules into scope, just like in Eta code. Observe that Data.List was added before the >.
In the REPL, it's nice to unimport modules as well, so there's a more powerful syntax using the :module command. You can use :m for shorthand.
You can add modules using :m +[module1] [module2] ... syntax.
Prelude Data.List> :m +Data.Monoid
Prelude Data.List Data.Monoid> "Hello" <> " " <> "World!"
"Hello World!"
it :: [Char]
You can remove modules with :m -[module1] [module2] ....
Prelude Data.List Data.Monoid> :m -Data.List
Prelude Data.Monoid> sort [1..10]
<interactive>:19:1: error:
Not in scope: 'sort'
Perhaps you meant 'sqrt' (imported from Prelude)
Prelude Data.Monoid> [1..10] <> [11..20]
[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
it :: (Enum t, Num t) => [t]
You can also specify exact what modules you want in scope with :m [module1] [module2] ....
Prelude Data.Monoid> :m Data.List Data.Char
Prelude Data.List Data.Char>
Note that Prelude is added to the list even though you didn't mention it. Because Prelude has all the commonly used methods, it is automatically added regardless of which :module command you use.