fromMaybe package:sbv

Return the value of an optional value. The default is returned if Nothing. Compare to fromJust.

>>> fromMaybe 2 (sNothing :: SMaybe Integer)
2 :: SInteger

>>> fromMaybe 2 (sJust 5 :: SMaybe Integer)
5 :: SInteger

>>> prove $ \x -> fromMaybe x (sNothing :: SMaybe Integer) .== x
Q.E.D.

>>> prove $ \x -> fromMaybe (x+1) (sJust x :: SMaybe Integer) .== x
Q.E.D.