Suppose we want to make a very simple REPL mode which just takes whatever input we provide and returns its quoted Expr form. We first make a parsing function,

julia> using ReplMaker

julia> function parse_to_expr(s)
           quote Meta.parse($s) end
       end
test_parser (generic function with 1 method)

Now, we can simply provide that parser to the initrepl function

julia> initrepl(parse_to_expr, 
                prompt_text="Expr> ",
                prompt_color = :blue, 
                start_key=')', 
                mode_name="Expr_mode")
REPL mode Expr_mode initialized. Press ) to enter and backspace to exit.

As instructed, we simply press the ) key and the julia> prompt is replaced

Expr>  

and as desired, we now can enter Julia code and be shown its quoted version.

Expr> 1 + 1
:(1 + 1)

Expr> x ^ 2 + 5
:(x ^ 2 + 5)

Next, we might notice that if we try to do a multiline expression, the REPL mode craps out on us:

Expr> function f(x)
:($(Expr(:incomplete, "incomplete: premature end of input")))

This is because we haven't told our REPL mode what constitues a valid, complete line. Since this REPL mode is just concerned with julia code, let's first make a function to detect if a string will parse to an incomplete expression.

julia> iscomplete(x) = true
iscomplete (generic function with 1 method)

julia> function iscomplete(ex::Expr)
           if ex.head == :incomplete
               false
           else
               true
           end
       end
iscomplete (generic function with 2 methods)

and then we can slurp up the string being stored in the REPL buffer, parse it and check if it is a complete expression:

julia> using REPL: LineEdit

julia> function valid_julia(s)
           input = String(take!(copy(LineEdit.buffer(s))))
           iscomplete(Meta.parse(input))
       end
valid_julia (generic function with 1 method)

Now all we have to do is redefine our REPL mode to use this completion checker:

julia> initrepl(parse_to_expr,
                prompt_text="Expr> ",
                prompt_color = :blue,
                start_key=')',
                mode_name="Expr_mode",
                valid_input_checker=valid_julia)
┌ Warning: REPL key ')' overwritten.
└ @ ReplMaker ~/.julia/packages/ReplMaker/pwo5w/src/ReplMaker.jl:86
REPL mode Expr_mode initialized. Press ) to enter and backspace to exit.

Expr> function f(x)
          x + 1
      end
:(function f(x)
      #= none:2 =#
      x + 1
  end)

This is an example of using a custom REPL mode to not change the meaning of the input code but instead of how results are shown. Suppose we have our own show-like function which is just Base.show, but will print Vectors and Tuples backwards

backwards_show(io, M, x) = (show(io, M, x); println(io))
backwards_show(io, M, v::Union{Vector, Tuple}) = (show(io, M, reverse(v)); println(io))

We can make a quick and dirty REPL mode that uses this rather than Base.show directly:

julia> initrepl(Meta.parse,
                show_function = backwards_show,
                prompt_text = "reverse_julia> ",
                start_key = ')',
                mode_name = "reverse mode")
REPL mode reverse mode initialized. Press ) to enter and backspace to exit.

reverse_julia> x = [1, 2, 3, 4]
4-element Array{Int64,1}:
 4
 3
 2
 1

The printing was reversed, but we can check to make sure the variable itself was not:

julia> x
4-element Array{Int64,1}:
 1
 2
 3
 4

For performance reasons, Julia defaults to 64 bit precision but sometimes you don't care about speed and you don't want to juggle the limitations of 64 bit precision in your head. You could just start wrapping every number in your code with big but that sounds like something the REPL should be able to do for you. Fortunately, it is!

using ReplMaker 

function Big_parse(str)
    Meta.parse(replace(str, r"[\+\-]?\d+(?:\.\d+)?(?:[ef][\+\-]?\d+)?" => x -> "big\"$x\""))
end

julia> initrepl(Big_parse, 
                prompt_text="BigJulia> ",
                prompt_color = :red, 
                start_key='>', 
                mode_name="Big-Mode")
REPL mode Big-Mode initialized. Press > to enter and backspace to exit.

Now press > at the repl to enter Big-Mode

BigJulia>  factorial(100)
93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

BigJulia>  factorial(100.0)
9.332621544394415268169923885626670049071596826438162146859296389521759999323012e+157

BigJulia>  factorial(100.0)^2
8.709782489089480079416590161944485865569720643940840134215932536243379996346655e+315

The package LispSyntax.jl provides a string macro for parsing lisp-style code into julia code which is then evaluated, essentially creating a lispy language embedded in julia.

julia> lisp"(defn fib [a] (if (< a 2) a (+ (fib (- a 1)) (fib (- a 2)))))"
fib (generic function with 1 method)

julia> lisp"(fib 30)"
832040

Awesome! To make this really feel like its own language, it'd be nice if it had a special REPL mode, so let's make one. For this, we're going need a helper function valid_sexpr to tell ReplMaker if we pressed return because we were done writing our input or if we wanted to write a multi-line S-expression.

using LispSyntax, ReplMaker
using REPL: REPL, LineEdit; using LispSyntax: ParserCombinator

lisp_parser = LispSyntax.lisp_eval_helper

function valid_sexpr(s)
  try
    LispSyntax.read(String(take!(copy(LineEdit.buffer(s)))))
    true
  catch err
    isa(err, ParserCombinator.ParserException) || rethrow(err)
    false
  end
end

Great, now we can create our repl mode using the function LispSyntax.lisp_eval_helper to parse input text and we'll use valid_sexpr as our valid_input_checker.

julia> initrepl(LispSyntax.lisp_eval_helper,
                valid_input_checker=valid_sexpr,
                prompt_text="λ> ",
                prompt_color=:red,
                start_key=")",
                mode_name="Lisp Mode")
REPL mode Lisp Mode initialized. Press ) to enter and backspace to exit.

λ> (defn fib [a] 
    (if (< a 2) 
      a 
      (+ (fib (- a 1)) (fib (- a 2)))))
fib (generic function with 1 method)

λ> (fib 10)
55

Tada!