Optional Values and Narrowing

An optional value is a union with none. Test its member type before using the contained value.

find = (present:bool):>int64|none => { if present { return 42 }; return none }
let result = find(true)
if result is? int64 { printl(result) }

Expected output:

42

Run the example from a file with dewy example.dewy. Its exact output is checked by the website build on the native release target.

Design boundary. The Optional Values and Narrowing reference specifies the complete language rules. A passing example covers this program only. See generated coverage for related acceptance programs and unmeasured areas.