Re: [stack] sweetening concatenative syntax

On Mar 6, 2008, at 5:36 PM, William Tanksley, Jr wrote:

> John Nowak <[email protected]> wrote:
>> William Tanksley, Jr wrote:
>>> ...but you're still doing patternmatching. Aren't you? What am I
>>> missing?
>> 'unlist' is just a normal function that gets defined as a result of
>> declaring the 'list' data type:
>>    unlist :: A {b} [A -> C] [A b {b} -> C] -> C
>
> Sorry, I have no idea how to read this. It's clearly a type signature,
> but I don't know any such languages more recent than ML (well, I know
> a little Scala), and that only from a few college assignments.
>
> I assumed that it was somewhat like 'i' in Joy (due to the name
> 'unlist' being similar to Joy's technical term 'dequote').

I explained what 'unlist' does in an earlier email. You may've missed  
it.

The 'unlist' function is not like 'i' in Joy. Joy's 'i' has the  
following type:

    i :: A [A -> B] -> B

In other words, given a stack and a function that transforms that  
stack to another stack, it yields the transformed stack.

What 'unlist' does is take three arguments. The first is a list. The  
second is a function that says what to do if that list is null. The  
the third is a function that acts on the list after is has been  
unconsed if it is not null. The 'unlist' function is called a sum  
deconstructor as lists are sum types (they're either a "null" or a  
"cons").

We can use 'unlist' to write the usual functions:

    uncons = [error] [] unlist
    head   = [error] [pop] unlist
    tail   = [error] [popd] unlist
    null?  = [true] [cons false] unlist

- John