LtU Forum

Ambiguous language namespaces

It seems like every single language that has come out recently has a name already in use in someway: swift, go, elm.

So I'm wondering, how horrible would it be to call a new language "Make" or "Maker"?

Compositional let bindings

I have been working on compositional let bindings, and wanted to get some comments on what I have so far. Each program fragment is typed with a monomorphic input context (the free variable requirements for the fragment) and an output polymorphic context (the definitions exported from the fragment). Lambda abstraction works as before, removing a variable from the input-context of the rhs. Let is more interesting, it adds the defined variable to the output-context, but we also want it to be usable in expressions. My first attempt is to have the let binding act as the identity function, so that apply does the necessary substitutions (symmetrically) from definitions in one fragment to requirements in the other. This is what a typing derivation looks like:

(x = \z . (z, z)) (x 1, x true)

1. [var]		{z : a} |- z : a
2. [var]		{z : a} |- z : a
3. [prd (1) (2)]	{z : a, z : b} |- (z, z) : (a * b)
4. [abs z (3)]		|- (\z . (z, z)) : (a -> (a * a))
5. [let x (4)]		|- {|- x : (a -> (a * a))} (x = (\z . (z, z))) : (b -> b)
6. [var]		{x : a} |- x : a
7. [lit]		|- 1 : Int
8. [app (6) (7)]	{x : (Int -> a)} |- (x 1) : a
9. [var]		{x : a} |- x : a
10. [var]		|- true : Bool
11. [app (9) (10)]	{x : (Bool -> a)} |- (x true) : a
12. [prd (8) (11)]	{x : (Int -> a), x : (Bool -> b)} |- ((x 1), (x true)) : (a * b)
13. [app (5) (12)]	|- {|- x : (a -> (a * a))} ((x = (\z . (z, z))) ((x 1), (x true))) : ((Int * Int) * (Bool * Bool))

Does this approach seem reasonable? It seems that I can implement all sorts of weird scoping rules, as the definitions compose upwards, which is probably not desirable but can be fixed by clearing the polymorphic output context where appropriate.

Lambda: A Peek Under the Hood

Brian Goetz presented
Lambda: A Peek Under the Hood
on October 31st 2013

I found the interaction between the implementation and language design for lambdas interesting.
Its nice that the open ended solution with Invoke dynamic avoided over specification and allows for a good selection of future optimizations.

Artificial Intelligence

Does anyone cares about learning some of AI methods?

here is an 11 pages paper

Making implicits less powerful?

Many people feel that Scala implicits are powerful enough to replace all uses of Haskell type classes, but might be too powerful, because they make it too easy to shoot yourself in the foot.

The problem is caused by not just implicit values, but also implicitly invoked functions. These are necessary if you want to cover all the uses of type classes. For example, the statement "if values of type T can be compared, then values of type List<T> can be compared as well" corresponds to an implicitly invoked function that receives a Comparator<T> and returns a Comparator<List<T>>. Having implicitly invoked functions introduces an alternate model of computation into the language, allowing a simple form of logic programming. A similar thing happens in Haskell with type class constraints. The Agda designers decided against implicitly invoked functions for precisely this reason.

I was unable to find any consensus on how to make implicits weaker, to allow useful scenarios but prevent harmful ones. Scala has adopted several such restrictions, described in the links above. I feel that the Scala model might be still too permissive. Here's some more ideas for restrictions:

1) Require explicit importing of all implicit values defined in other modules, except the ones automatically defined by the language (e.g. Comparator<Pair<Int,Int>> or the function Comparator<T> -> Comparator<List<T>>). This might be too drastic though, especially if the language requires you to say "deriving Eq" and doesn't define stuff automatically.

2) Check for all possible ambiguities in implicit resolution, not just actual ones. Haskell's rules for instance contexts (7.6.3) are one way to make that decidable. In Scala I guess it's undecidable, but can't say for sure.

3) Allow only types whose definition is marked as "implicit" to be used as implicit arguments. For example, Comparator would be allowed to be implicit, but Int or String or function types wouldn't. This would also remove a potential ambiguity, because functions marked as "implicit" would be always treated as implicitly invoked functions, and never as implicit values.

Has anyone here given this some thought? What are the engineering considerations?

2014 APL Programming Competition is Open

The sixth annual International APL Problem Solving Competition is now live!

Dyalog Ltd invites students worldwide to put their programming and problem-solving skills to the test by using any APL system to develop solutions to ten questions and solve a series of problems. This is a contest for people who love a challenge and learning new things for fun, with the added bonus that you can win one of 43 cash prizes totalling $8,500, including a grand prize of $2,500 and a trip to Eastbourne in the U.K. to attend the annual Dyalog Ltd user meeting in September 2014.

For the rules and eligibility criteria and to enter the competition, go to

If you have friends who love a challenge and learning new things for fun, or you know students who might be interested in participating, then please recommend this contest to them.

The deadline for submitting solutions is 6 August 2014. Winners will be announced on 18 August 2014.

Good luck and have fun!

Notation for debugging type derivations.

I want to produce traces of the type derivations for compositional type inference to help debugging programs. Currently the reports look like this:

var ---------------------------------------
x : {x : a} |- a

var ---------------------------------------
y : {y : a} |- a

x : {x : a} |- a
y : {y : a} |- a
app ---------------------------------------
(x y) : {x : (a -> b), y : a} |- b

var ---------------------------------------
z : {z : a} |- a

(x y) : {x : (a -> b), y : a} |- b
z : {z : a} |- a
app ---------------------------------------
((x y) z) : {x : (a -> (b -> c)), y : a, z : b} |- c

However repeating the typings in each derivation seems a bit of a waste of space. I am thinking I can just reference which rule is used and number the statements, a bit like a theorem proof. Something like:

1) var |= x : {x : a} |- a
2) var |= y : {y : a} |- a
3) app (1) (2) |= (x y) : {x : (a -> b), y : a} |- b
4) var |= z : {z : a} |- a
5) app (3) (4) |= ((x y) z) : {x : (a -> (b -> c)), y : a, z : b} |- c

Does this seem a reasonable notation? it the use of the double-turnstile "|=" a misuse of notation? Is there a better way to write this?

states in stateless machine

Hi all!

I'm experimenting with constructing a programming language for artificial intelligence programming. I want it also to be enabled for regular application programming. Right now i have thought through the stateless part, meaning that I have a rough definition of underlying stateless machine.

In real world there are also constructs that require statefull machines to hold some knowledge, but I'd hate to extend current language definition with additional statefull stuff that complicates the language that is now so simple without it.

So, I'm wondering are there known methods or new ideas to describe states inside stateless machines?

Tx :)

Project Zonnon: Oberon for distributed programming in the large

Contains odors of: Oberon and Actor Model type (E, Erlang, etc.) languages.

The Concepts of Zonnon:

  • design support for ‘programming in the large’
  • a modern object model that supports concurrency: activities in objects replacing passive method calls
  • formalised interaction between activities (dialogs)
  • good for structured and OO programming styles for a wide range of applications … OS to Business
  • retain the simplicity of PASCAL, Modula-2, Oberon family of languages
  • support multi-processor and distributed systems
  • support inter-operation with other languages and their libraries
  • produce a .NET Compiler, IDE, Test Suite and concise Language Report (40 Pages)

Some things: Facets: Allowing objects to not be forced into one major role; Syntax based 'dialogs' govern concurrent interaction; Object level concurrency/threads; Statement level concurrency; 40 page language specification.

When I skim the slides and see the code and statements about the approach to concurrency, it looks to me off the cuff that it would just make deadlock even bigger and more hairy. :-}

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