Showing and Running Code
.code, .play and friends
9 October 2026
A whole file
.code code/hello/main.go shows a file as it is:
package main
import "fmt"
func main() {
fmt.Println("Hello, 世界")
}
2
Part of a file
An address picks the lines to show. Lines ending in OMIT are left out:
func fib(n int) int {
a, b := 0, 1
for range n {
a, b = b, a+b
}
return a
}
A regular expression alone picks a single line:
3
Highlighting
Lines ending in // HLstep are highlighted when the command ends in HLstep:
func fib(n int) int {
a, b := 0, 1
for range n {
a, b = b, a+b
}
return a
}
Each word marks its own group of lines. This file also has // HLmain:
func main() {
for i := range 10 {
fmt.Print(fib(i), " ")
}
fmt.Println()
}
4
Line numbers and editing
func main() {
for i := range 10 {
fmt.Print(fib(i), " ")
}
fmt.Println()
}
-edit lets the audience change the code on the slide:
package main
import "fmt"
func main() {
fmt.Println("Hello, 世界")
}
5
Running code locally
The rest of these slides run programs with .play. That needs present running
on your own machine: pages written by present export, such as these examples
on GitHub Pages, only show the code, without a Run button.
To run the programs, start present locally:
go run github.com/some-programs/present@latest serve -doc
Then open http://127.0.0.1:3999 and choose Examples.
6
Running Go
.play runs the program on the machine running present. Click Run, or press shift+enter:
package main
import "fmt"
func main() {
fmt.Println("Hello, 世界")
}
7
Running only part of the source
Only the selected lines are shown, but the whole file is run:
func main() {
jobs := make(chan int)
results := make(chan string)
for w := range 3 {
go worker(w, jobs, results)
}
go func() {
for i := range 6 {
jobs <- i
}
close(jobs)
}()
for range 6 {
fmt.Println(<-results)
}
}
8
Several files
A txtar archive holds several files; without a go.mod, one is made.
-- go.mod --
module example/multi
go 1.27
-- main.go --
package main
import (
"fmt"
"example/multi/greet"
)
func main() {
fmt.Println(greet.Hello("present"))
}
-- greet/greet.go --
package greet
func Hello(name string) string {
return "Hello from another package, " + name + "!"
}
9
Compile errors
Only main is shown, but the errors' line numbers are those of the file, the one in count too:
func main() {
n := 1
fmt.Println(greeting(), n+"!")
fmt.Println(missing)
}
10
Running Python
A .py file runs with python3:
print(list(itertools.islice(primes(), 20)))
11
Running JavaScript and TypeScript
.js files run with node, .ts files too once tsc has checked and compiled them:
for (let i = 1; i <= 15; i++) {
console.log((i % 3 ? '' : 'Fizz') + (i % 5 ? '' : 'Buzz') || i);
}
function area(s: Shape): number {
switch (s.kind) {
case 'circle':
return Math.PI * s.radius ** 2;
case 'square':
return s.side ** 2;
}
}
12
Other languages
A program starting with #! runs with that interpreter:
#!/bin/sh
# Programs starting with #! run with that interpreter instead of Go.
echo "Running on $(uname -s) as $(whoami)"
date
13
Source of slide 1S closes
examples/code.slide
1 # Showing and Running Code
2 .code, .play and friends
3 9 Oct 2026
4
5 Ada Example
6 ada@example.com
Source of slide 2S closes
examples/code.slide
8 ## A whole file
9
10 `.code code/hello/main.go` shows a file as it is:
11
12 .code code/hello/main.go
File used
code/hello/main.go 1 package main
2
3 import "fmt"
4
5 func main() {
6 fmt.Println("Hello, 世界")
7 }
Source of slide 3S closes
examples/code.slide
14 ## Part of a file
15
16 An address picks the lines to show. Lines ending in `OMIT` are left out:
17
18 .code code/fib/main.go /START OMIT/,/END OMIT/
19
20 A regular expression alone picks a single line:
21
22 .code code/fib/main.go /^func main/
File used
code/fib/main.go 1 package main
2
3 import "fmt"
4
5 // START OMIT
6 func fib(n int) int {
7 a, b := 0, 1
8 for range n {
9 a, b = b, a+b // HLstep
10 }
11 return a
12 }
13
14 // END OMIT
15
16 func main() {
17 for i := range 10 {
18 fmt.Print(fib(i), " ") // HLmain
19 }
20 fmt.Println()
21 }
Source of slide 4S closes
examples/code.slide
24 ## Highlighting
25
26 Lines ending in `// HLstep` are highlighted when the command ends in `HLstep`:
27
28 .code code/fib/main.go /^func fib/,/^}/ HLstep
29
30 Each word marks its own group of lines. This file also has `// HLmain`:
31
32 .code code/fib/main.go /^func main/,/^}/ HLmain
File used
code/fib/main.go 1 package main
2
3 import "fmt"
4
5 // START OMIT
6 func fib(n int) int {
7 a, b := 0, 1
8 for range n {
9 a, b = b, a+b // HLstep
10 }
11 return a
12 }
13
14 // END OMIT
15
16 func main() {
17 for i := range 10 {
18 fmt.Print(fib(i), " ") // HLmain
19 }
20 fmt.Println()
21 }
Source of slide 5S closes
examples/code.slide
34 ## Line numbers and editing
35
36 .code -numbers code/fib/main.go /^func main/,/^}/
37
38 `-edit` lets the audience change the code on the slide:
39
40 .code -edit code/hello/main.go
Files used
code/fib/main.go 1 package main
2
3 import "fmt"
4
5 // START OMIT
6 func fib(n int) int {
7 a, b := 0, 1
8 for range n {
9 a, b = b, a+b // HLstep
10 }
11 return a
12 }
13
14 // END OMIT
15
16 func main() {
17 for i := range 10 {
18 fmt.Print(fib(i), " ") // HLmain
19 }
20 fmt.Println()
21 }
code/hello/main.go 1 package main
2
3 import "fmt"
4
5 func main() {
6 fmt.Println("Hello, 世界")
7 }
Source of slide 6S closes
examples/code.slide
42 ## Running code locally
43
44 The rest of these slides run programs with `.play`. That needs present running
45 on your own machine: pages written by `present export`, such as these examples
46 on GitHub Pages, only show the code, without a Run button.
47
48 To run the programs, start present locally:
49
50 go run github.com/some-programs/present@latest serve -doc
51
52 Then open http://127.0.0.1:3999 and choose Examples.
Source of slide 7S closes
examples/code.slide
54 ## Running Go
55
56 `.play` runs the program on the machine running present. Click Run, or press shift+enter:
57
58 .play code/hello/main.go
59
60 : Run present serve -play=false to turn off running programs.
File used
code/hello/main.go 1 package main
2
3 import "fmt"
4
5 func main() {
6 fmt.Println("Hello, 世界")
7 }
Source of slide 8S closes
examples/code.slide
62 ## Running only part of the source
63
64 Only the selected lines are shown, but the whole file is run:
65
66 .play -edit code/pipeline/main.go /^func main/,/^}/
File used
code/pipeline/main.go 1 package main
2
3 import (
4 "fmt"
5 "time"
6 )
7
8 func main() {
9 jobs := make(chan int)
10 results := make(chan string)
11
12 for w := range 3 {
13 go worker(w, jobs, results)
14 }
15
16 go func() {
17 for i := range 6 {
18 jobs <- i
19 }
20 close(jobs)
21 }()
22
23 for range 6 {
24 fmt.Println(<-results)
25 }
26 }
27
28 func worker(id int, jobs <-chan int, results chan<- string) {
29 for j := range jobs {
30 time.Sleep(200 * time.Millisecond)
31 results <- fmt.Sprintf("worker %d did job %d", id, j)
32 }
33 }
Source of slide 9S closes
examples/code.slide
68 ## Several files
69
70 A [txtar](https://pkg.go.dev/golang.org/x/tools/txtar) archive holds several files; without a go.mod, one is made.
71
72 .fontsize 0.9
73
74 .play code/multi.txtar
File used
code/multi.txtar 1 -- go.mod --
2 module example/multi
3 go 1.27
4 -- main.go --
5 package main
6
7 import (
8 "fmt"
9
10 "example/multi/greet"
11 )
12
13 func main() {
14 fmt.Println(greet.Hello("present"))
15 }
16 -- greet/greet.go --
17 package greet
18
19 func Hello(name string) string {
20 return "Hello from another package, " + name + "!"
21 }
Source of slide 10S closes
examples/code.slide
76 ## Compile errors
77
78 Only `main` is shown, but the errors' line numbers are those of the file, the one in `count` too:
79
80 .play -numbers code/_broken/main.go /^func main/,/^}/
File used
code/_broken/main.go 1 // This program does not compile, to show that the line numbers in the
2 // compiler's errors match the ones on the slide. It is in a directory
3 // starting with _ so that the go command leaves it out of ./...
4 package main
5
6 import "fmt"
7
8 func greeting() string {
9 return "hello"
10 }
11
12 func main() {
13 n := 1
14 fmt.Println(greeting(), n+"!")
15 fmt.Println(missing)
16 }
17
18 func count() int {
19 return "not a number"
20 }
Source of slide 11S closes
examples/code.slide
82 ## Running Python
83
84 A `.py` file runs with `python3`:
85
86 .play -edit code/primes.py /^print/
File used
code/primes.py 1 import itertools
2
3
4 def primes():
5 """Yields the prime numbers, using the sieve of Eratosthenes."""
6 composites = {}
7 for n in itertools.count(2):
8 if n not in composites:
9 yield n
10 composites[n * n] = [n]
11 continue
12 for p in composites.pop(n):
13 composites.setdefault(n + p, []).append(p)
14
15
16 print(list(itertools.islice(primes(), 20)))
Source of slide 12S closes
examples/code.slide
88 ## Running JavaScript and TypeScript
89
90 `.js` files run with `node`, `.ts` files too once `tsc` has checked and compiled them:
91
92 .play code/fizzbuzz.js
93
94 .play code/shapes.ts /^function area/,/^}/
Files used
code/fizzbuzz.js 1 for (let i = 1; i <= 15; i++) {
2 console.log((i % 3 ? '' : 'Fizz') + (i % 5 ? '' : 'Buzz') || i);
3 }
code/shapes.ts 1 type Shape =
2 | { kind: 'circle'; radius: number }
3 | { kind: 'square'; side: number };
4
5 function area(s: Shape): number {
6 switch (s.kind) {
7 case 'circle':
8 return Math.PI * s.radius ** 2;
9 case 'square':
10 return s.side ** 2;
11 }
12 }
13
14 const shapes: Shape[] = [
15 { kind: 'circle', radius: 1 },
16 { kind: 'square', side: 2 },
17 ];
18 for (const s of shapes) {
19 console.log(s.kind, area(s).toFixed(2));
20 }
Source of slide 13S closes
examples/code.slide
96 ## Other languages
97
98 A program starting with `#!` runs with that interpreter:
99
100 .play code/system.sh
File used
code/system.sh 1 #!/bin/sh
2 # Programs starting with #! run with that interpreter instead of Go.
3 echo "Running on $(uname -s) as $(whoami)"
4 date
No sourceS closes
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