Fuzion • Library Modules • base • choice • apply

choice.apply

type

, F

type

apply -- apply `f.call` to the value stored in this choice

This can be used to perform operation on values of arbitray choice types.

Example:

# This is a feature that can be called with an argument of any choice
# type with two type parameters, i.e, two variants.
#
reflect_on_choice(X, Y type,
                  C type : choice X Y,
                  c C)
=>
  c.apply T,v->
    if T : numeric then
      say "c is numeric $v, c² is $(v*v)"
    else if T : String then
      say "c is String '$v' length $(v.codepoint_count)"
    else
      say "c is of type $T value $v"
# NYI: ENHANCEMENT: #7791 better type inference should avoid the explicit types here:
reflect_on_choice false_ true_ bool true
reflect_on_choice String nil (option String) "hi!"
reflect_on_choice String nil (option String) nil
reflect_on_choice (num.complex f64) (num.fraction i64) (choice (num.complex f64) (num.fraction i64)) (num.complex -1.0 1.0)
reflect_on_choice (num.complex f64) (num.fraction i64) (choice (num.complex f64) (num.fraction i64)) (3⁄4)

What are effects?

which prints

Type Parameters

0.099dev (GIT hash a68bce9af280552faedee34cd5433dc10b6454de)

last changed: 2026-10-14