I once used it for mathematical code in my thesis. Basically I had everything written with C++ code but it just didn't work. Also tried Matlab but I'm just not made to use it efficiently, so I tried Haskell for interfacing with the GNU GSL library. In the end I changed back to C++ but it helped me a great deal to understand what I was doing and to clean things up. Some nice features include obviously dealing only with constant expressions and being forced to write only mathematical transformations. A bit unexpected was that you can actually swap variable declarations, declaration order doesn't seem to matter much. Quite a big pain relieve when dealing with a lot of variables.

Actually ImplicitCAD is written in Haskell which implements CSG (very tricky to efficiently implement)