| u2b | MPS.Extra, MPS, MPSUTF8 |
| ub | MPS.Light, MPS.Env, MPS, MPSUTF8 |
| uncurry | MPS.Env |
| undefined | MPS.Env |
| unique | MPS.Light, MPS.Env, MPS, MPSUTF8 |
| unlines | MPS.Env |
| until | MPS.Env |
| unwords | MPS.Env |
| unzip | MPS.Env |
| unzip3 | MPS.Env |
| upper | MPS.Light, MPS.Env, MPS, MPSUTF8 |
| upto | MPS.Light, MPS.Env, MPS, MPSUTF8 |
| userError | MPS.Env |