[OpenSCAD] Extending OpenSCAD from assembler to C or Perlish language and adding standard library

nop head nop.head at gmail.com
Tue Oct 1 11:41:55 EDT 2019


Thanks for the detailed explanation.  I never program anything complicated
enough in OpenSCAD to need anything mutable variables.

On Tue, 1 Oct 2019 at 16:09, Doug Moen <doug at moens.org> wrote:

> On Tue, Oct 1, 2019, at 3:02 AM, nop head wrote:
>
> People say it is hard to write algorithms without mutable variables. Does
> that mean it is hard to write algorithms in Haskell or does that have them
> as well?
>
>
> Haskell and OpenSCAD are at opposite ends of the spectrum, in terms of
> expressive power. It's true that Haskell and OpenSCAD are both declarative
> languages with immutable variables, but aside from that, they are totally
> different.
>
> Haskell has many features and idioms which are not available/not possible
> in OpenSCAD, which are used as an alternative to mutable variables.
>
> 0. Haskell has generalized tail recursion optimization. The corresponding
> feature in OpenSCAD is restricted to one specific code pattern, and only
> works for self-recursive functions, not mutually recursive functions.
>
> 1. Haskell has combinators, which are functions that take functions as
> arguments, and return functions as results. There is a large standard
> library of combinators, which are used as functional control structures,
> and replace most uses of tail recursion. These library combinators use tail
> recursion internally, and encapsulate high level patterns of tail
> recursion. Tail recursion is considered to be a "low level" coding pattern,
> which you only resort to when necessary.
>
> This style of programming will become possible in OpenSCAD once function
> values are added to the language. To fully support this style of
> programming, we would want better tail recursion support, and a very terse
> syntax for anonymous function literals.
>
> 2. Haskell has lazy data structures. Notably, it has lazy lists. A lazy
> list is a potentially infinite list, in which list elements are only
> computed the first time you access them, then the element values are
> cached. Using lazy lists, you can decompose an imperative loop into two
> pieces: the piece that generates the sequence of elements is encapsulated
> as a lazy list. The piece that consumes the sequence of elements is now
> separate. This enables a new kind of modularity and a new style of
> programming.
>
> 3. Haskell has monads, which are a generalization of sequentially executed
> statements in imperative programming. In OpenSCAD, you can use sequences of
> if statements and for statements at the top level, or inside a module, to
> construct an "implicit union" of shapes. In Haskell terminology, we would
> call that the "union monad". In OpenSCAD, you can also use this same
> statement syntax inside a list comprehension, to construct a sequence of
> list values. This corresponds to the List Monad in Haskell. Haskell has
> many other monads, and it's easy to define new ones.
>
> One of the things that originally impressed me about OpenSCAD is the fact
> that it has monads (which are declarative) dressed up to look exactly like
> C statements and control structures (which are imperative). In Curv, I've
> created something that is essentially a "mutable variable monad". It looks
> just like imperative programming, but it is syntactic sugar for something
> that is actually declarative and referentially transparent.
>
> Haskell monads are notoriously difficult to learn (due to the way they are
> presented). In F#, the same feature is called "computation expressions".
> The syntax and presentation of computation expressions is much more
> friendly to imperative programmers. In F#, the "list monad" is called the
> "sequence builder", and F# sequence expressions are very similar to
> OpenSCAD list comprehensions, except that there are more statement types,
> including a "while" statement.
>
> 4. In imperative programming languages, you use mutable variables to
> incrementally build up a compound data structure. You can assign an
> individual component of an array using `a[i] = newvalue`, and this is an
> efficient operation. Haskell has functional data structures, which support
> efficient incremental update without mutation, and Haskell has an "optics"
> library for efficiently constructing and composing get and set operations
> on individual elements of a large data structure. Think of optics as a
> declarative query language.
>
> I think that function values are an essential programming feature, and
> they will allow us to cherry pick some of the easiest to understand idioms
> from Haskell. But I also think that imperative style programming idioms are
> easier for the OpenSCAD community than the corresponding Haskell idioms
> that are used for doing the same job.
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