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Playing with the REPL


The documentation that you're reading is a design document where most of the features you're reading are yet to be implemented. Check the Note on the Docs

Even though NeoHaskell is a compiled language, it provides an interpreted mode which is much faster for the development process. In any moment, you can run neo run:repl and it will start an interactive console where you can checkout small pieces of NeoHaskell code.

trade offer meme with a repl

When you run the command, it will start, what's called in programmer jargon, a REPL (Read-Eval-Print-Loop). A REPL will do the following algorithm:

  1. Read the code you wrote
  2. Evaluate (execute) it
  3. Print the result
  4. Loop back to 1

Let's fire up neo run:repl:


This is the prompt of the REPL. It's waiting for you to write some code. Let's write our first hello world program!

neo> print "Hello World!"
Hello World!

Awesome, we've completed our first hello world program under a minute. Let's try doing some math:

neo> 1 + 1

neo> 2 * 2

neo> 7 ** 20

Whoops, the last number is a bit too big. This is because ** is the exponentiation operator. It's the same as writing 7207^{20} in math.

Anyway, to quit the REPL, you can press Ctrl + C or write :quit.

Exploring the REPL commands

At any time, you can write :help to see the list of commands available.

Don't worry too much about them if they seem a bit overwhelming. You'll learn them as you go.

In the next sections, we will start trying NeoHaskell from the REPL.