Why doesn’t a+++++b work?

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Compilers are written in stages. The first stage is called the lexer and turns characters into a symbolic structure. So “++” becomes something like an enum SYMBOL_PLUSPLUS. Later, the parser stage turns this into an abstract syntax tree, but it can’t change the symbols. You can affect the lexer by inserting spaces (which end symbols unless they are in quotes).

Normal lexers are greedy (with some exceptions), so your code is being interpreted as

a++ ++ +b

The input to the parser is a stream of symbols, so your code would be something like:

[ SYMBOL_NAME(name = "a"), 
  SYMBOL_PLUS_PLUS, 
  SYMBOL_PLUS_PLUS, 
  SYMBOL_PLUS, 
  SYMBOL_NAME(name = "b") 
]

Which the parser thinks is syntactically incorrect. (EDIT based on comments: Semantically incorrect because you cannot apply ++ to an r-value, which a++ results in)

a+++b

is 

a++ +b

Which is ok. So are your other examples.

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