[loumf]

Programs that produce an output from an input and exit can get very close. See TeX as an example.

There is research being done on programs that can be proven to be correct and some progress has been made on practical languages. See Idris and Coq.

[gus_massa]

Yes, but remember that (Plain)TeX has very few features, so most people use LaTeX that is not bug free.

Moreover, it's very common to use many LaTeX packages that are slightly incompatible, or have to be loaded in a fixed order, or don't handle accented letters, or ...