Tutorial: a tiny JSON HTTP service in Go 1.26 using new(expression)

By Ada Brackett — Solnyxus editorial voice. Drafted with AI assistance; reviewed by the team.

This is the durable, step-by-step companion to the blog post on Go 1.26's new(expression). Written for Go 1.26.0; tested on Linux (amd64) on 14 October 2026.

What you will build#

A single-file web service listening on 127.0.0.1:8080 with one route, GET /reading, that returns JSON. The temp_c field is a *float64 set with new(21.5), a form that is valid from Go 1.26.

Prerequisites#

• A terminal and a text editor.
• curl for testing.
• Go 1.26 or newer, or Go 1.21+ able to download toolchains automatically (see below).
• Port 8080 free on your machine.

1. Install or select Go 1.26#

Check what you have:

go version

If it prints go1.26 or higher, continue. If you have Go 1.21 or newer but older than 1.26, you can let Go fetch the right toolchain by prefixing commands with an environment variable:

export GOTOOLCHAIN=go1.26.0
go version

The first run downloads the toolchain; later runs reuse it. Otherwise install Go from the official download page at go.dev/dl.

2. Create the module#

mkdir tiny && cd tiny
go mod init tiny

Open go.mod and ensure the version line reads go 1.26 (adjust it if go mod init wrote an older value):

module tiny

go 1.26

3. Write the code#

Save as main.go:

package main

import (
	"encoding/json"
	"errors"
	"log"
	"net/http"
	"time"
)

type Reading struct {
	Sensor string   `json:"sensor"`
	Temp   *float64 `json:"temp_c"`
	At     string   `json:"at"`
}

func readingHandler(w http.ResponseWriter, r *http.Request) {
	rd := Reading{
		Sensor: "kitchen",
		Temp:   new(21.5),
		At:     time.Now().UTC().Format(time.RFC3339),
	}
	w.Header().Set("Content-Type", "application/json")
	if err := json.NewEncoder(w).Encode(rd); err != nil {
		log.Println("encode:", err)
	}
}

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("GET /reading", readingHandler)
	srv := &http.Server{Addr: "127.0.0.1:8080", Handler: mux, ReadHeaderTimeout: 5 * time.Second}
	log.Println("listening on", srv.Addr)
	if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
		log.Fatal(err)
	}
}

Notes: new(21.5) allocates a float64 initialised to 21.5 and returns *float64. The "GET /reading" pattern restricts the route to GET (method patterns exist since Go 1.22).

4. Check it compiles#

gofmt -l .
go vet ./...
go build -o tiny .

gofmt -l prints nothing when formatting is right; go vet and go build print nothing on success.

5. Run it#

./tiny

You should see a log line listening on 127.0.0.1:8080. (Or use go run ..)

6. How to know it worked#

In a second terminal:

curl -i localhost:8080/reading

Expect HTTP/1.1 200 OK, Content-Type: application/json, and a body like:

{"sensor":"kitchen","temp_c":21.5,"at":"2026-10-14T16:06:23Z"}

The timestamp will differ. Then confirm the method restriction:

curl -i -X POST localhost:8080/reading

Expect 405 Method Not Allowed. Stop the server with Ctrl+C.

Common errors#

Variations#

• Return nil for Temp to see "temp_c":null.
• Replace new(21.5) with a function call, e.g. new(readSensor()), since any expression is allowed.

Reference#

Go 1.26 Release Notes, section "Language": go.dev/doc/go1.26 (checked 14 October 2026).