My experience has been that you can get incredibly far (much farther than you'd think) without any form of AST and just iterate over the raw parse tree and toke

My experience has been that you can get incredibly far (much farther than you'd think) without any form of AST and just iterate over the raw parse tree and tokens, but the problem you eventually encounter when you do this is that if you want to support all the different cases in a 'real' and messy evolved language like C is that you get a combinatorical exposion when you try to support all the corner cases of the language. A great example of this would be if you try to figure out how to write a function to iterate over the tokens that make up a struct definition in C: http://blog.robertelder.org/magical-world-of-structs-typedefs-scoping/ http://blog.robertelder.org/magical-world-of-structs-typedef... This is the reason for needing the extra 'abstract' syntax tree layer that lets you generalize over all those different cases and avoid having 20-level nested if statements in your compiler.