Diet Coke

A small functional language with effect inference based on Koka.

Quick Setup (macOS)

brew install sbt rlwrap

Requirements

Diet Coke is built using Scala 2.11.7 and depends only on Scala Parser Combinators. In order to make the most use of it, you should have sbt installed, and the repl.sh also depends on rlwrap.

Using Diet Coke

Diet Coke consists of both a single file interpreter and a REPL. The interpreter simply takes a path to a single file and interprets it (n.b. this requires an entrypoint named main). The REPL is a fairly straightforward console, featuring only three commands (:multiline, :type, and :quit). You can toggle multiline input mode on and off using :m on and :m off (or the long-form :multiline). You can query the type of an arbitrary expression using :type expr or :t expr for short. Finally, you can quit the console using :quit or :exit. The REPL will also bind the result of every computation to a fresh result variable. To run a single file, use sbt "run file.coke" and to run the REPL, use repl.sh.

Syntax

expr ::= id
       | n
       | true | false
       | string
       | op1 expr | expr op2 expr
       | builtIn(expr*) | expr(expr*)
       | error string
       | fix id => expr | id => expr
       | if expr then expr else expr
       | { expr [; expr]* }

builtIn ::= show
          | print
          | println
          | catch
          | inject
          | random

op1 ::= ! | -

op2 ::= + | - | * | / | %
      | == | /= | > | < | >= | <= |
      | && | || | ^

typ ::= () | num | bool | string | id | typ -> typ

stmt ::= let id = expr
       | fn id(id*) = expr
       | expr
       | stmt; stmt
       | type id :: typ

Example Programs with REPL Sessions

Program

let x = 10
x * x

Session

λ let x = 10
λ x * x
let res4: num = 100

Program

fn randRange(x) = random() % x

Session

λ fn randRange(x) = random() % x
λ :t randRange
randRange :: (num -> <ndet | a> num)
λ randRange(5)
let res5: num = -4
λ randRange(10)
let res6: num = 8

Program

fn randRangeErr(x) =
  if x < 0 then 
    error "invalid input"
  else
    random() % x

Session

λ fn randRangeErr(x) = if x < 0 then error "invalid input" else random() % x
λ :t randRangeErr
randRangeErr :: (num -> <ndet, exn | a> num)
λ randRange(10)
let res7: num = 8
λ randRange(32)
let res8: num = -27

Program

fn fact(x) =
  if x < 0 then
    error "undefined"
  else if x == 0 then
    1
  else
    x * fact(x - 1)

Session

λ :m on
λ fn fact(x) =
|   if x < 0 then
|     error "undefined"
|   else if x == 0 then
|     1
|   else
|     x * fact(x - 1)
| 
λ :m off
λ :t fact
fact :: (num -> <exn | a> num)
λ fact(5)
let res15: num = 120
λ fact(10)
let res16: num = 3628800

Program

fn trace(x) = ({ println(show(x)); x })()

fn wild(x) =
  if x == 0 then
    error "bad"
  else if x > 0 then
    trace(14)
  else
    random()

Session

λ fn trace(x) = ({ println(show(x)); x })()
λ :t trace
trace :: (a -> <io | b> a)
λ trace(30)
30
let res9: num = 30
λ :m on
λ fn wild(x) =
|   if x == 0 then
|     error "bad"
|   else if x > 0 then
|     trace(14)
|   else
|     random()
| 
λ :m off
λ :t wild
wild :: (num -> <ndet, io, exn | a> num)
λ wild(10)
14
let res10: num = 14
λ wild(10)
14
let res11: num = 14
λ wild(0)
(0): bad
λ wild(-32)
let res12: num = 2027454890
λ wild(-32)
let res13: num = -536856153

Miscellaneous

To-Do List

• Fix bug with typing inject and catch builtins.
• Fix bug causing type errors to be order-dependent in multi-statement files.
• Implement explicit effect annotations.

Full Citation

Daan Leijen. Koka: Programming with row polymorphic effect types. 
In Paul Levy and Neel Krishnaswami, editors, Proceedings 5th Workshop on 
Mathematically Structured Functional Programming, MSFP 2014, Grenoble, 
France, 12 April 2014, pages 100–126, 2014.