@api.get '/v1/image/:format' do
    width = Valuer.value Integer, params['width'], default: 1280, min: 1, max: 4000
    height = Valuer.value Integer, params['height'], default: 900, min: 1, max: 10_000
    delay = Valuer.value Float, params['delay'], default: 0.0, min: 0.0, max: 10

more examples

require "valuer"

Valuer.value Integer, "20"
=> 20

Valuer.value Integer, "20x"
=> Valuer::Error (invalid value for Integer(): "20x")

Valuer.value Integer, "20x", default: -1
=> -1

Valuer.value Integer, "20", min: 21
=> Valuer::MinimumError (minimum is 21)

Valuer.value Integer, "20", min: 19
=> Valuer::MaximumError (maximum is 19)

Valuer.value Float, "1.2", min: 1, max: 20
=> 1.2

Valuer.value TrueClass, "yes"
=> true

Valuer.value TrueClass, "no"
=> false