Lesson 21 · Recomposing a URI into a string

Recomposing a URI into a string

Parsing takes a string apart; today you do the reverse, reassembling the five components back into a URI string. This recomposition is what lets you print a resolved reference, and it is where the present-versus-absent flags finally pay off.

The goal

Recompose a URI's components back into a string, following RFC 3986 Section 5.3.

Start here - the target

TO DO

Scenario: Component recomposition

Giventhe URI parsed from "http://a/b/c/d;p?q"

Whenit is recomposed with String

Thenthe result is exactly "http://a/b/c/d;p?q"

AndParse("p?").String() is "p?" and Parse("path#").String() is "path#", because an empty-but-present query or fragment still writes its delimiter

Background

Recomposition is the inverse of the Chapter 1 split, and RFC 3986 Section 5.3 spells out the exact procedure: write the scheme and its colon if there is a scheme, then // and the authority if there is an authority, then the path, then ? and the query if there is a query, then # and the fragment if there is a fragment. Reassembled in that order, a parsed URI round-trips back to its original text.

This is where the Has... booleans earn their keep. The delimiter is written based on presence, not on whether the string is non-empty: p? has an empty but present query, so recomposition must emit the ?, and path# must emit the trailing #. If you tested Query != "" instead of HasQuery, both delimiters would vanish and the round-trip would break. Getting recomposition exact matters immediately, because reference resolution produces a new URI as five components and the only way to read out an absolute URL - the thing the whole project builds toward - is to recompose it into a string.

Make it work

// RFC 3986 5.3: put back each delimiter only if the part is present
func (u *URI) String() string {
  // scheme + ":" if scheme != ""
  // "//" + authority if HasAuthority
  // path
  // "?" + query if HasQuery
  // "#" + fragment if HasFragment
}

CheckpointDONE

A URI recomposes back into the exact string it was parsed from. Commit and stop here.