5. State as a library

Teaches: State is a handler, not a keyword

There is no mutable-variable keyword in bang. state 0 in … installs a handler that interprets get/put as reads/writes against a seeded value — mutability is ordinary library code over the same effect-and-handler mechanism every other effect in this tour uses (kernel invariant: the only privileged primitive is STM, and even that ships as a handler in v1).

state 0 in let c = {get}; z = put 5 in $c

Expected output (bang run stdout):

5

Run it yourself:

curl -fsSL https://raw.githubusercontent.com/phibkro/bang/main/tools/install.sh | sh
bang run examples/state/main.bang

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