Interactive programming (repl)

Many languages come with a command line interpreter or shell.

Show how to start the interpreter, then, as a small example of its use, interactively create a function of two strings and a separator that returns the strings separated by two concatenated instances of the separator.

For example, f('Rosetta', 'Code', ':') should return 'Rosetta::Code'

Note: this task is not about creating your own interpreter.

Common Lisp

The details of interactive use vary widely between implementations; this example is from SBCL. * is the prompt. By default, SBCL compiles (not interprets) all code, unless sb-ext:*evaluator-mode* is changed.

$ rlwrap sbcl
This is SBCL 1.0.25, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.
...
* (defun f (string-1 string-2 separator)
    (concatenate 'string string-1 separator separator string-2))

F
* (f "Rosetta" "Code" ":")

"Rosetta::Code"
*

E

<lang sh>$ rune # from an OS shell. On Windows there is also a desktop shortcut.</lang>

"?" and ">" are prompts for input; "#" marks output.

<lang e>? def f(string1 :String, string2 :String, separator :String) { > return separator.rjoin(string1, "", string2) > }

value: <f>

? f("Rosetta", "Code", ":")

value: "Rosetta::Code"

</lang>

If you type a definitely incomplete expression, such as "def f() {", then it gives an ">" prompt and takes additional lines. If the expression is not necessarily incomplete, you can continue anyway by ending a line with "\".

Haskell

The details of interactive use vary widely between implementations. This example is from GHCi.

$ ghci
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.4.2, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base-1.0 ... linking ... done.
Prelude> let f as bs sep = as ++ sep ++ sep ++ bs
Prelude> f "Rosetta" "Code" ":"
"Rosetta::Code"

OCaml

Because you can enter expressions that span multiple lines, you have to type the double semicolon (";;") at the end so that it knows you are done.

$ ocaml
        Objective Caml version 3.10.2

# let f s1 s2 sep = String.concat sep [s1; ""; s2];;
val f : string -> string -> string -> string = <fun>
# f "Rosetta" "Code" ":";;
- : string = "Rosetta::Code"

Python

Start the interpreter by typing python at the command line (or select it from a menu). You get a response showing the version of the interpreter being run before giving an input prompt of three greater-than characters and a space:

<lang python>python Python 2.6.1 (r261:67517, Dec 4 2008, 16:51:00) [MSC v.1500 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> def f(string1, string2, separator): return separator.join([string1, , string2])

>>> f('Rosetta', 'Code', ':') 'Rosetta::Code' >>> </lang>

Ruby

Start the interpreter by typing irb at the command line. You will see an input prompt, which by default is name of this program(name of main object):line number:indent level> :

<lang ruby>$ irb irb(main):001:0> def f(string1, string2, separator) irb(main):002:1> [string1, , string2].join(separator) irb(main):003:1> end => nil irb(main):004:0> f('Rosetta', 'Code', ':') => "Rosetta::Code" irb(main):005:0> </lang>