The rose apple is also known as the Malay apple, water apple, and mountain apple in English, chompu in Thai and Khmer, and jambu in Malay. It is a member of the Myrtaceae family and is prized for its pretty trees, flowers, and fruit. Native to Southeast Asia, it now grows in South America as well.
I know the rose apple from Thailand, where I was first introduced to it, along with a host of other weird fruits. The ones that I ate were bell-shaped, and had thin, shiny, smooth, pale green or pink skin; spongy, mild, slightly sweet flesh; and a few dark, hard seeds in the hollow centre. Apparently this type is known as the long-fruited rose apple to distinguish if from round and oval varieties. While some find them oddly insipid, I like their mild watery flavour and texture. On a hot day I find 1 or 2 quite refreshing, though I wouldn't eat more than that.