Concrete syntax is how the code looks to the programmer. The abstract syntax are the syntax trees that result from parsing the concrete syntax.
In our interpreters, the abstract syntax are the expression case classes (e.g.
App(funName: Symbol, ...) ). The expressions build expression trees that are then used by the interpreters and type checkers. We did not care much about concrete syntax: the programs are written with the abstract syntax.
In Slide V02 p.4, we use a grammar to define how the abstract syntax trees of some language look like (we then introduce case classses to model these trees in Scala). In Slide V03 p.8, we use a grammar to define how the programmer is writing code. A programmer would write
instead of
In essence:
Concrete syntax = what the programmer writes
Abstract syntax = what the interpreter sees (result of parsing concrete syntax)
