Yes! Having functions of the schematic form
f(x: a >> b) : c
is one of the most amazing and original features of ATS. I remember my hair
literally standing on end when I first saw it.
Very much looking forward to ATS3!
lördag 5 juni 2021 kl. 15:36:24 UTC+2 skrev gmhwxi:
> FYI.
>
> Functional programming (FP) puts emphasis on "purity" these days.
> In particular, it preaches all sorts of benefits gained from avoiding
> updates in programming. This somewhat leaves one with the impression
> that "pure" programs cannot have updates. How wrong! A novelty of ATS
> precisely lies in that "pure" programs can have updates.
>
> I am from the camp of FP. As a matter of fact, the core of ATS is
> ML-like; it supports type inference, polymorphism, higher-order
> functions, datatypes and pattern matching, etc. However, I also want
> to stress that ATS offers much more than the usual kind of ML-like
> functional programming, which, in my opinion, is far from being
> adequate for systems programming (that is, implementing low-level
> systems).
>
> Once upon a time, we had LISP machines and operating systems written
> in LISP. I suppose that most people would call LISP a functional
> language. But LISP is not ML-like; it does not preach "purity" at
> all. In particular, typical LISP programs make pervasive use of
> updates. Maybe we should just refer to LISP as a functional imperative
> programming (FIP) language.
>
> What is so attractive about FIP? Let us first see a serious issue
> with ML-like functional programming. With functional data structures
> (that do not support modifications), it is often difficult, if
> possible at all, to give an efficient tail-recursive implementation of
> certain simple recursive functions. For instance, a famous example is
> the following 'append' for concatenation two lists:
>
> fun
> append
> (xs, ys) =
> (
> case xs of
> | nil() => ys
> | cons(x1, xs) => cons(x1, append(xs, ys))
> )
>
> Assume that the direct style of compilation is used to compile
> 'append'. Since 'append' is recursive but not tail-recursive, each
> recursive call to 'append' requires allocating a new frame on the call
> stack. This simply implies that calling 'append(xs, ys)' on a long
> list 'xs' requires a deep stack for otherwise it causes the call stack
> to overflow.
>
> One possible attempt to implement 'append' in a tail-recursive manner
> leads to the following terribly inefficient implementation:
>
> fun
> append
> (xs, ys) =
> rappend(reverse(xs), ys)
> {
> fun
> reverse(xs) = rappend(xs, nil)
> and
> rappend
> (xs, ys) =
> (
> case xs of
> | nil() => ys
> | cons(x1, xs) => rappend(xs, cons(x1, ys))
> )
> } (* end of [append] *)
>
> Note that the implementation of 'rappend' is tail-recursive. The
> tail-recusive version of 'append(xs, ys)' traverses 'xs' once to
> construct its reverse and then traverses the reverse to perform
> reverse-appending; the constructed reverse of 'xs' becomes garbage at
> the end.
>
> While using non-tail-recursion is probably fine in the construction of
> a compiler, it is not the case in systems programming, where it is not
> uncommon to see the requirement that only tail-recursive functions
> (that is, loops) be allowed.
>
> ATS offers much more than the usual ML-like FP. It supports so-called
> linearly typed FIP where (linear) data structures can be updated in a
> type-safe manner. With it, one can write in ATS type-safe programs
> that are very close to their counterparts in C in terms of both
> functionalities and performances. Also, ATS supports type-safe manual
> memory management of C-style, obviating the need for a big run-time
> memory management system if the programmer so chooses.
>
>
>
> For previously post messages:
>
> https://github.com/githwxi/ATS-Xanadu/tree/master/docgen/NOTES
>
> #
>
>
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