Lesson 01 · The scheme - Build an HTTP Client

The scheme

Every request starts from a URL, and the very first thing a client reads off it is the scheme - http or https - which decides how to talk to the far end. Today you build the smallest possible URL parser, one that pulls just the scheme, so the type every later lesson thickens exists from day one.

The goal

Parse a URL string into a value whose scheme field holds the lowercased scheme.

Start here - the target

TO DO

Scenario: Reading the scheme off a URL

Giventhe URL string "http://example.com"

Whenit is parsed

Thenthe parsed URL has scheme "http"

Andparsing "HTTPS://example.com" gives scheme "https" (schemes are lowercased)

Background

A URL is the address a client aims at, and its first component is the scheme: the http or https before the ://. The scheme is the single most important thing about a URL because it decides everything downstream - which default port to use, whether the connection is encrypted, how the request is framed. So the parser earns its keep by reading the scheme first.

Schemes are case-insensitive, so HTTP, Http, and http all mean the same thing; the convention is to store them lowercased so later comparisons are simple equality checks. Keep the type small today - a struct with one filled field and room for the rest. Everything else about the URL arrives one lesson at a time on top of this.

Make it work

// the whole client will grow around this type
type URL struct {
  Scheme string
  // Host, Port, Path, Query, Fragment arrive over the next lessons
}
// the scheme is everything before the first "://", lowercased
func Parse(raw string) (*URL, error) {
  // split on "://", lowercase the left half
}

CheckpointDONE

You can construct a URL value and read its scheme, lowercased. Commit and stop here.