README.md

Stafeful computation with the State effect

The State[S, ?] models a stateful computation that receives a current state S, and emits a new state S along with the payload.

We will use State to try to account for the effect of symlinks in our file traversal. If we traverse a directory tree that contains multiple symlinks to the same target file, we won't count the size of the target file more than once.

To do this we'll need to keep track if what files we've seen before. This is what requires the use of a State effect. In this case, the state we'll be tracking (ie type S) will be Set[FilePath]

Tasks

🔍 Study Code

• Note that FilePath now includes a new case Symlink(path: String, linkTo: FilePath) and the DefaultFilesystem uses the File API to distinguish symlinks. Previously they were automatically followed.

📝 Write Code

• Define an alias _sym[R] in EffTypes to indicate that State[Set[FilePath], ?] is a member of effect stack R

• Add the member constraint to pathScan and scanReport

• We need to interpret the state effect in main. We'll use the evalStateZero[Set[FilePath]] combinator. Eval here means that the final state isn't returned, just the computed payload. The zero suffix indicates that the zero value of a Monoid[Set[FilePath]] in scope should be used as the initial state. That's coming in from imports cats.implicits._. The zero value of a set is Set.empty.

• In pathScan, you'll should add a case to handle Symlink. To read/write the current state use the get/put combinators, and/or modify(f: S => S) to run a state modification function. Your logic should check if the target of a symlink has been visited; if it has, then return an empty pathscan, while if it hasn't, add it to the visited set and invoke pathScan on the target.

  This is a tricky task. Remember to look at the solution for guidance if you get stuck.

▶️ Run Code

• Run the test to check that symlinks are being handled.