When writing programs that take other programs as inputs, and/or produce other programs as outputs, it's tempting to treat the program as only slightly more structured than its textual representation.

The problem is that unless your use case is very limited and is guaranteed to stay that way, supporting more and more language constructs will quickly turn your code into a mess.

Compiler design as we learn it (lex/parse, syntax tree, semantic checks, transforms, lowering to codegen) is _the_ solution to the problem of dealing with computer programs as inputs and outputs. Trying to do something less is like solving a dynamic programming problem without knowing dynamic programming: it will only work for a restricted set of inputs.