Marlowe in Blockly

So far in these tutorials we have concentrated on building contracts in the textual version of Marlowe, embedded in Haskell, Marlowe contracts can also be built using the Blockly visual programming environment, as we describe here.

Getting started

To start building a Blockly project in the Marlowe Playground, which you can find out more about in the section about playground overview. On the home screen

Playground landing page

select Start in Blockly. You’ll now see a screen like this:

Blockly window

Contracts are assembled by adding components to the holes in the blocks. To build a contract, we have to fill the single, top-level, hole here in the CONTRACT block with a Contract: you can see that it filled with a Close here.

Blocks are selected by clicking on them: the currently selected blocks are outlined in yellow, as here:

Selected blocks in the Blockly window

Once a block or blocks are selected you can move them directly; cut, copy and delete them using your operating system’s usual short cuts; and access other operations by right-clicking on them.

Suppose that you remove the Close block and you want to build another contract. The block templates for contracts are found by clicking on Contracts in the menu on the left hand side. Doing this shows all the different blocks that build contracts, as shown here:

Contracts in Blockly

We can select one and drag it into the editing area, which has a dotted background, like this:

A contract block: When

Looking at the menu on the left hand side, you can see two new entries below the original: these tell you what kind of block you need to fit into each of the holes in the currently selected block (that is the block outlined in yellow).

In the example here you can see that the upper hole requires a case block while the lower one needs to be filled by another contract.

Let’s fit the contract into the top-level hole, so that we’re constructing a contract with a When as its main construct.

Fitting in a contract block

We can then repeat this process, adding contracts and other types of components – all of which are shown in the menu on the left-hand side – to build a complete contract, that is a contract that contains no holes.

Let’s fill in the timeout information and add a Close contract to be performed in the case of timeout:

Adding timeout information

What action should we choose now? Again we see all the possible actions by selecting case or Actions in the menu:

The Action menu

If we select a Deposit action we have a number of different types of holes to fill.

Making a Deposit

Here we have to choose

  • A from_party for the depositor,

  • A value, which is the amount deposited …

  • … in which currency, or token (often ada)

  • Into which account, given by a party, which is either a role or a public key.

  • The continuation contract.

In filling in some of these we have also to fill in a text field, or a number, as shown in the final result here.

Completing a Deposit contract

Finally, we have to decide what the continutation contract is: what does the contract do next? That’s another contract. Here it is completed as a payment:

The complete contract


In Blockly, complete for yourself the construction of the contract above.

What is the effect of the example contract? In particular, what are the payments made by the contract?

Editing Blockly

Blockly contracts can be manipulated and edited using visual gestures and keyboard short cuts.

Right-click menu
  • Blocks can be dragged from holes, as well as being inserted.

  • There is a right click menu available, as seen in the image above, offering a range of options.

  • Typical editing short cuts are operative, e.g. DEL, ⌘C, ⌘V, and ⌘X on Mac OS.