Weekly Assignments 7

To be submitted: Tuesday, 22 September 2020, 12:30 MNG

Note that some tasks may deliberately ask you to look at concepts or libraries that we have not yet discussed in detail. But if you are in doubt about the scope of a task, by all means ask.

Please try to write high-quality code at all times! This means in particular that you should add comments to all parts that are not immediately obvious. Please also pay attention to stylistic issues. The goal is always to submit code that does not just correctly do what was asked for, but also could be committed without further changes to an imaginary company codebase.

W7.1 Static Analysis

We know that in Marlowe, it is not an error to reference a ValueId that has not been set to a value.

Write a static analyzer

undefinedValueIds :: Contract -> Set ValueId

that statically detects such uninitialized ValueIds for a given contract and returns them as a set.

Note that if a ValueId has been initialized in one branch (like one branch of an If or one Case of a When), but is used in another branch, it should still be included in the list of results.

W7.2 Crowd Sourcing Campaign

Write a function

crowd :: Amount   -- ^ campaign goal
      -> Amount   -- ^ individual contribution
      -> Role     -- ^ campaign owner
      -> [Role]   -- ^ contributors
      -> Slot     -- ^ deadline
      -> Contract

which create a Marlowe contract for a crowd-sourcing campaign.

In such a campaign, the campaign owner sets a campaign goal and asks for contributions to his campaign. If the contributions exceed the goal when the deadline is reached, the campaign was successful, and the owner gets all contributed funds.

If on the other hand the contributed funds are lower than the goal, the contributors get reimbursed once the deadline is reached.

In the function crowd, we use a fixed list of potential contributors and a fixed amount that each contributor can contribute. Contributors can contribute only once, and they should be able to contribute at any time before the deadline (so they do not have to wait for other contributors to make a contribution).

For example, if the goal is 1000 ada, the fixed individual contribution is 500 ada and there are three contributors, then there are four possible outcomes of the campaign:

• Nobody contributes. The campaign fails, and nothing happens.
• One contributor contributes 500 ada. The goal of 1000 ada has not been reached, the campaign fails, and the contributor gets her 500 ada back.
• Two contributors contribute 500 ada each. The goal of 1000 ada has been reached (just barely), the campaign was successful, and the campaign owner gets 1000 ada.
• All three contributors contribute. The goal has been exceeded, the campaign was successful, and the campaign owner gets 1500 ada.

Use the simulator to test your contract for at least the above example and those four cases.