This is an Oriental fruit-fly. You can see the halteres in pic #2. For those not in the know, halteres are the 2nd pair of wings (or what's left of them anyway). The diff btw bees and flies are bees have 2 pairs of wings while flies have one pair and the halteres. This particular fruitfly is considered a pest and their larvae are usually responsible for destroying fruits, and in the past have been known to cause huge losses to fruit farms all over. I guess agriculture is not a big thing in urban Singapore, so this fly is not considered a pest here.
Sorry Sgt Pepper, it is an Oriental Fruitfly and not a melon fly. Altho they are in the same genus, D. dorsalis is different from D. cucurbitae
Look here for pix of D.cucurbitae. Have seen enough of the Oriental fruitfly to know one when I see it.