Lesson 03 · Filter mode - Build a Fuzzy Finder

Filter mode

With a match test in hand, the tool can finally do its job. Today you wire matching into the entry point so a query on the command line filters the candidate stream down to the lines that match.

The goal

Add a filter mode that, given a query argument, prints only the candidates that match it, preserving input order.

Start here - the target

TO DO

Scenario: Filtering the stream by a query

Giventhe input lines "src/main.go", "README.md", "go.mod", "src/app.go" and the query "go"

Whenthe program runs in filter mode with that query

Thenit prints "src/main.go", "go.mod", and "src/app.go" in that (input) order, and omits "README.md"

Andan empty query prints every input line unchanged

Background

This is the first lesson where the tool earns its name. The skeleton from lesson 1 streamed every line; now a query turns it into a filter. Running it with go over a file list keeps only the paths where g, o appear in order and drops the rest. This non-interactive filter mode is exactly fzf’s -f flag, and it stays useful for the whole project: it is how you drive the pipeline from a script or a pipe, without a terminal UI.

Preserving input order matters here. You are not ranking yet - that comes later - so the filtered lines should appear in the same relative order they arrived. Keeping filtering and ranking as separate steps is what lets you get a working tool today and layer match quality on top without rewriting this loop.

Make it work

// A -f/--filter flag (or the first positional arg) carries the query.
// Reuse matches() as the gate; print a line only when it passes.
query := os.Args[...]  // read the query argument
for sc.Scan() {
  line := sc.Text()
  if matches(query, line) {
    fmt.Println(line)
  }
}

CheckpointDONE

The tool now filters real input down to fuzzy matches. Commit and stop here.