# on
[](https://hex.pm/packages/on)
[](https://hexdocs.pm/on/)
Pattern-matching in functional form on the core Gleam types.
```sh
gleam add on@1
```
#### Example 1
```gleam
import gleam/float
import gleam/int
import gleam/option.{type Option, None, Some}
import gleam/string
import on
type CSSUnit {
PX
REM
EM
}
fn extract_css_unit(s: String) -> #(String, Option(CSSUnit)) {
use <- on.true_false(
string.ends_with(s, "rem"),
on_true: #(string.drop_end(s, 3), Some(REM)),
)
use <- on.true_false(
string.ends_with(s, "em"),
on_true: #(string.drop_end(s, 2), Some(EM)),
)
use <- on.true_false(
string.ends_with(s, "px"),
on_true: #(string.drop_end(s, 2), Some(PX)),
)
#(s, None)
}
fn parse_to_float(s: String) -> Result(Float, Nil) {
case float.parse(s), int.parse(s) {
Ok(number), _ -> Ok(number)
_, Ok(number) -> Ok(int.to_float(number))
_, _ -> Error(Nil)
}
}
pub fn parse_number_and_optional_css_unit(
s: String,
) -> Result(#(Float, Option(CSSUnit)), Nil) {
let #(before_unit, unit) = extract_css_unit(s)
use number <- on.ok(parse_to_float(before_unit))
Ok(#(number, unit))
}
```
#### Example 2
```gleam
import gleam/io
import gleam/string
import on
import simplifile
fn read_file(path: String) -> Result(String, String) {
on.error(
simplifile.read(path),
on_error: fn(e) { Error("simplifile FileError: " <> string.inspect(e)) },
)
}
pub fn main() -> Nil {
use contents <- on.error_ok(
read_file("./sample.txt"),
on_error: fn(msg) { io.println("\n" <> msg) }
)
use first, rest <- on.lazy_empty_nonempty(
string.split(contents, "\n"),
on_empty: fn() { io.println("empty contents") },
)
use <- on.lazy_false_true(
string.trim(first) == "<!DOCTYPE html>",
on_false: fn() { io.println("expecting DOCTYPE in first line") },
)
use parse_tree <- on.error_ok(
parse_html(rest),
on_error: fn(e) { println("parse error: " <> string.inspect(e)) },
)
// ...
}
```
Further documentation can be found at <https://hexdocs.pm/on>.
## Development
```sh
gleam run # Run the project
gleam test # Run the tests
```
