chickadee » srfi-146 » mapping-key-predecessor

mapping-key-predecessor mapping obj failureprocedure
mapping-key-successor mapping obj failureprocedure

Returns the key contained in the mapping mapping that immediately precedes/succeeds obj in the mapping's order of keys. If no such key is contained in mapping (because obj is the minimum/maximum key, or because mapping is empty), returns the result of tail-calling the thunk failure.