An interactive Ruby lesson

Blocks, then closures

Tell Ruby what to do. Then find out what that code remembers.

New to Ruby? Start at the beginning. If you already use blocks, jump to closures.

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Part 1: blocks

A block is a piece of code passed to a method. Here, each visits every name and runs the block for each one.

1. Say hello

Predict the output. Then run the code. Try changing a name or the message.

Ruby code: greetings

Show expected output

Hello, Ana!
Hello, Ben!
Hello, Cora!

do ... end marks the block. |name| names the current item. each handles iteration; the block supplies the action.

2. Collect results with map

Predict: does this print three separate lines or one array?

Ruby code: map

Show answer

One array: ["Hello, Ana!", "Hello, Ben!", "Hello, Cora!"]. map collects each block result in a new array.

3. Call a block yourself

A method can use yield to call the block it receives.

Ruby code: yield

Show expected output

Welcome to the meetup!
Glad you're here, Ana.

The method decides when to call the block. The caller decides what the block does. Try changing the block's message.

Part 2: closures

A closure retains access to variables from its surrounding scope. A lambda is a callable object that can carry that access beyond the method that created it. A block can also be captured as a Proc; here we use lambdas.

4. Two counters

Predict the four numbers. Do the counters share a count?

Ruby code: independent counters

Show answer

Each call to make_counter creates a separate count. Each lambda retains access to its own variable after the method returns.

5. Two closures, one variable

Predict what read prints before and after two increments.

Ruby code: shared counter

Show answer

0
2

Both lambdas retain access to the same variable. One changes it; the other reads it. Try making another pair. Does it share the first pair's count?

6. Carry configuration

A closure can carry a setting with behavior. Change the prefixes and names.

Ruby code: configurable greeting

Show expected output

Good evening, Ana!
Hey, Ana!

Keep learning

Make the greeting block print a different message for each name.
Add a decrement lambda to the shared counter. What does read show?
Read the Ruby documentation on blocks and Proc and lambda documentation.

The runner uses ruby.wasm. Browser Ruby has environment limits; these examples do not require Rails or a Ruby server.