So far, all of the blocks you've defined have been "command" blocks. These actual carry out an action that affects the state of the program. For example, the move block makes the sprite move, and the say block makes the sprite say something. We call these effects "side effects."

Sometimes, we aren't interested in any side effects, and just want a block that performs a calculation for us. These are called "reporters," because rather than causing the program to do something, they simply give us back a value. Many blocks on the "operators" palette are reporters. Here are just a few:

a portion of the 'operators' palette

Notice how all of these are rounded, and can't be attached directly to other blocks. Because of this, we need to use them in combination with other blocks, like say or set. For example:

say (4 + 7)

(I'm sure you've been doing things like this since you started using Snap!)

Let's learn how to make our own reporter blocks!

  1. The first step is, as usual, to select "make a block" from the bottom of the "variables" palette.
  2. Next, we'll select "reporter" rather than the usual "command."

    select reporter as block type

    Note: you don't have to necessarily put all reporters in the "operators" palette; it's just the most frequent place for them.
  3. Once you give your block a name and click "OK," your block definition will look something like this:

    empty reporter body

    Notice that report () has automatically been added to the block definition. This is to remind you that all reporter blocks must report a value.
  4. Fill in the body with whatever you want, and you're done!

Let's take a look at an example:

square(x) block definition

This block will take in a parameter x, and report x2.

Here's a more complex example, using if:

homemade absolute value block

This block is a home-made version of the abs (absolute value) block that we saw earlier. Notice that it contains two report blocks. Reporter blocks can have as many reports as you want/need.

Let's try writing our own reporter block! Try writing a max block that takes in two numbers and reports the larger of the two. Hint: you'll need to use if.

empty max(x, y) block definition

Here's a more complex block that we wrote. Predict what it will report when run with inputs "hello" and 5.

mysterious block definition

"Predicates" are special kinds of reporter blocks that only report true or false. You'll see in the next slide how to write them.