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Read, Execute, and Print Loops

REPLs have some unique requirements from standard compilation:

  • Maintaining scope across multiple invocations of the compiler.
  • Compiling small fragments of code (often expressions instead of statements).
  • Automatic processing of results (printing, format, history)

This document describes the issues and some general design ideas for their solution, but currently lacks concrete designs. It should be updated as implementing a PACT REPL gets closer to reality.


Generally, users of a REPL expect that functions and variables defined in one line to be available in successive lines. Compiling each line as a separate program means that all this information is lost. Therefore, some semblance of scope must be maintained through both the compiler and evaluation.

Users also generally expect to be able to redeclare variables created in the REPL (and possibly those outside, depending on language standards). The simplest way to model this behavior is that each line of the REPL creates a new scope embedded inside the scope from the previous line. To prevent this nesting from exploding memory requirements, flattening all previous scopes to a single "outer" scope for the next line is desired.


Issues of outer scopes need to be handled in compilers constantly. However, in this case, we have two particular needs. First, the compiler needs to accept an outer scope as an argument. Second, the compiler needs to output scope information in addition to the code. This output is what gets passed back into the compiler for the next line. This allows the compiler to generate variable and function accesses correctly across lines of the REPL.

In addition, the compiler will need to be certain not to store variables in registers as we'd like to be able to garbage collect contexts after each line. Likely the compiler should default to using lexical variables or storing variables in a context object.


Not only do the locations of variables and functions need to be preserved for the compiler, the actual data needs to be preserved for the user. The "Execute" portion of the REPL will need to handle setting up the context for the evaluated code. This includes ensuing that the correct subroutines are available, setting up the lexical environment, and possibly passing and saving a context object.

Custom Lexpad

Parrot's lexical operators (find_lex and store_lex) already use vtable functions to access the LexPad PMC. It may be useful to create a custom LexPad to store variables inside of a hash (or a hash per type) instead of registers. This makes handling variables in a REPL identical to handling lexical variables to the compiler and ensures that all data is updated properly.

Fragment Compilation

Generally a REPL is compiling far smaller units than a normal program. Users expect to able to write a single expression rather than a full program. Compiler writers will need to be able to handle that in their parsers and call the appropriate hooks from the REPL.

This is also related to the idea that the PAST::AST compiler should be very liberal in the code that it accepts. Given a bare expression, it should output an unnamed subroutine that runs the given code and returns the result. This implies that subroutines, the call to return, and other standard boilerplate are automatically handled by PACT. There should be hooks to make it simple for HLLs to customize that boilerplate as well.

Processing Results

Once the code has been compiled and run, the final section of the loop is to print the results. The REPL will have to determine the type and number of results and handle them appropriately. (Remember that PCC allows for return values to be as complex as parameters.)

Each return value needs to be converted to a string. There should be options for switching between the get_string VTABLE, dumper, and a custom function to format the output.

REPLs become even more useful when you can refer to old results (i.e. irb's _ and __[n]). Obviously the details of how it is accessed will vary per HLL, but the REPL framework should make storing and recalling this information easy.