Instructions

In this exercise you're going to write a generic (/ magical!) SecureTreasureChest, to store some treasure. Sharp eyed students will have noticed that the previous TreasureChest wasn't that secure! If you used the getTreasure function, then you had to supply the password, but you could still simply destructure the TreasureChest type to get the treasure, without having to know the password. Even worse, you could even retrieve the password in the same way! Let's fix that by using an Opaque Type.

1. Define the Password opaque type, using the "Parse, don't validate" pattern

Define a Password custom type with a single variant.

The variant should have an associated String value, for the password.

You should expose the Password type, but not the variant. This makes Password an Opaque type, and means that you can no longer use destructing to get the password. It is not possible to test whether a type is Opaque or not, in production code, you would use elm-review (a static analysis tool) to enforce it.

Define and expose a createPassword function, with a type definition, that takes a String and returns a Maybe Password.

Owasp recommends that passwords should be at least 8 characters long, so lets use the Parse, don't validate pattern to enforce this invariant.

If the password is less than 8 characters long, createPassword should return Nothing, otherwise it should return Just Password.

2. Expose SecureTreasureChest, but not it's variant

You should expose the SecureTreasureChest type, but not the variant. This makes SecureTreasureChest an Opaque type, and means that you can no longer use destructuring to get the password or the treasure.

3. Define the createTreasure function

This function should take two parameters, and have a type definition.

• a generic type (for the treasure)
• a Password (for the password)

The function should return a SecureTreasureChest.

Expose this function, so that other modules are able to create instances of SecureTreasureChest.

4. Define the getTreasure function

This function should take two parameters, and have a type definition.

• a String (for the password)
• a SecureTreasureChest generic custom type

This function should check the provided password attempt against the Password in the SecureTreasureChest.

The function should return a Maybe.

If the passwords match then return Just with the generic value from the SecureTreasureChest (the treasure!)

If the passwords do not match then return Nothing.

Expose this function, so that other modules are able to get the treasure from a SecureTreasureChest, as long as they supply the correct password.