• ← Testing Expressions
• → Tracing and Profiling

The Main Loop

An interactive session operates a loop, called the “main loop” in this way:

• read input
  
• process input
  
• format and print results
  
• repeat

As part of this loop, various global objects in this section are consulted.

There are a variety of “hooks” that allow you to insert functions to be applied to the expressions at various stages in the main loop.

If you assign a function to the global variable $PreRead it will be applied with the input that is read in the first step listed above.

Similarly, if you assign a function to global variable $Pre, it will be applied with the input before processing the input, the second step listed above.

• $HistoryLength — number of previous lines of input and output to keep
• $Post — function applied to each expression after evaluation
• $Pre — function applied to each input expression before evaluation
• $PrePrint — function applied after 'Out[n]' is assigned, but before the result is printed
• $PreRead — function applied to each input string before being fed to the Wolfram System
• $SyntaxHandler — function applied to any input line that yields a syntax error
• In — Kth input
• $Line — current line number