What Formatters Solve

A formatter rewrites source code in a canonical style. Same code, same formatting, every time.

input:    function f(x,y){return x+y}
output:   function f(x, y) {
            return x + y;
          }

Why?

• Style consistency across a team: nobody argues over braces.
• Readability: standard layout = familiar reading.
• Diff clarity: a function rename doesn't reformat the file.
• Speed: developers don't waste cycles aligning code.

Real formatters:

• gofmt (Go): canonical, no options. Universal.
• rustfmt (Rust): canonical with limited options.
• prettier (JS/TS/CSS/MD): opinionated, multi-language.
• black (Python): opinionated.
• clang-format (C/C++): highly configurable.

Two design philosophies:

1. No options (gofmt, black): one true style. Eliminates bikeshedding.
2. Many options (clang-format): match team's existing style.

Trade-off: gofmt achieved universal Go style; clang-format achieved pluralism.

Wadler's "A Prettier Printer" (2003): elegant algebra for pretty-printing structured data. Foundation of prettier, and many functional pretty printers.

We'll build a Wadler-style pretty printer + apply it to a small language. The formatter takes an AST and renders it as text within a width budget.

Compare to a linter:

• Linter: finds problems, suggests/applies fixes for specific issues.
• Formatter: rewrites layout (whitespace + line breaks) only.
• Both run in pre-commit / CI.