examples: add compiletime/d_compile_value.v (#21738)

examples/compiletime/d_compile_value.v

@@ -0,0 +1,56 @@
+// This example shows how to use V's compile time defines and values via the -d flag and $d() function.
+//
+// To change the default values below, pass compile flags to v using: `-d ident=<value>`
+// Examples:
+// ´´´
+// v -d header -d pad=10 -d jobs=8 run d_compile_value.v
+// v -d id="Bob" -d pad=2 -d pad_char='$' -d jobs=10 run d_compile_value.v
+// ```
+
+const pad = $d('pad', 5)
+const pad_char = $d('pad_char', `-`)
+
+const footer = '
+Available compile time flags:
+ -d pad=<i64>
+ -d pad_char=<character>
+ -d id="<string>"
+ -d jobs=<i64>
+ -d header=<bool>
+You can turn this message off with:
+ -d footer=false'
+
+struct Job {
+	id string = $d('id', 'Job') // adjust with `-d id="My ID"`
+}
+
+struct App {
+	jobs [$d('jobs', 4)]Job // adjust fixed array size with `-d jobs=6`
+}
+
+// println_padded prints `str` to stdout with a padding.
+// Padding length and padding character can be adjusted at compile time
+// via `-d pad=1` and `-d pad_char=x`.
+fn println_padded(str string) {
+	for _ in 0 .. pad {
+		print(rune(pad_char))
+	}
+	println(' ${str}')
+}
+
+fn main() {
+	header := $d('header', false) // adjust header variable with `-d header=true` or simply `-d header`
+	if header {
+		println_padded('Compile Value Example')
+	}
+
+	app := App{}
+	for i, job in app.jobs {
+		println_padded('${job.id} ${i + 1}')
+	}
+
+	// Turn the footer message off using `-d footer=false`
+	if $d('footer', true) {
+		println(footer)
+	}
+}