[General] Definition of the variables inside an interval

Quick question: can we define a variable on a given interval instead of R?
For example:
`
a \in [0, 1], variable
`

If I impose
`
a \in R, variable
`
and
`
a >= 0
`
and
`
1 - a >= 0
`
as constraints, the optimizer can still pick a outside of [0,1] even though it doesn't satisfy the constraint. The problem is that there are some functions in my model which are not defined for an a outside of [0,1], what makes the optimizer crash.