## Saturday, November 24, 2012

### Rev 3.7 release with map/reduce; Python-like syntax

Revision 3.7 of the alpha release 0.5 of the ParaSail compiler and virtual machine is now available at the same URL we have been using recently:

http://bit.ly/Mx9DRb

This release has a number of new features, including a special syntax for map/reduce style computations, filters added to iterators, and support for an optional Python-like syntax.  For a description and some examples of the new map/reduce syntax, see:

http://parasail-programming-language.blogspot.com/2012/11/special-syntax-for-mapreduce-in-parasail.html

Here is a simple example using the map/reduce syntax (and a filter) to compute the square of N by summing the first N odd integers:
```    func Square(N : Univ_Integer {N >= 0}) -> Univ_Integer is
return (for I in 1 ..< 2*N {I mod 2 == 1} => <0> + I);
end func Square;
```
Filters are mentioned in the above blog entry, and illustrated in the above example. Here is another example where a "functional" version of QSort uses filters with a triplet of container comprehensions.  The filters are immediately before the "=>", enclosed in {...}:
```
func QSort(Vec : Vector<Element>) -> Vector<Element> is
if |Vec| <= 1 then
return Vec;  // The easy case
else
const Mid := Vec[ |Vec|/2 ];  // Pick a pivot value
return
QSort( [for each E of Vec {E < Mid} => E] )  // Recurse
| [for each E of Vec {E == Mid} => E]
| QSort( [for each E of Vec {E > Mid} => E] ); // Recurse
end if;
end func QSort;
```
Note that we have added a "magnitude" operator "|...|" which can be defined as appropriate for a given interface; for vectors |V| is equivalent to Length(V).

The ParaSail compiler now recognizes a Python-like variant, in addition to the "normal" ParaSail syntax. In the Python-like syntax variant, "is", "then", "of", and "loop" are replaced by ":"; semicolons are optional; "end if", "end case", etc. are not used. "end class", "end interface, "end func" are optional. Indenting is significant. There are also some Python-inspired synonyms: "def" may be used instead of "func"; "elif" may be used instead of "elsif"; "# " may be used instead of "//" to introduce a comment (notice the required space).

The Python-like syntax is best illustrated by example. Here is the same QSort function given above using the Python-like syntax:
```
def QSort(Vec : Vector<Element>) -> Vector<Element>:
if |Vec| <= 1:
return Vec  # The easy case
else:
const Mid := Vec[ |Vec|/2 ]  # Pick a pivot value
return
QSort( [for each E of Vec {E < Mid} => E] )  # Recurse
| [for each E of Vec {E == Mid} => E]
| QSort( [for each E of Vec {E > Mid} => E] )  # Recurse
```
Note that the above is strongly typed (unlike Python), but because of type inference on declarations, types only show up in the parameter and result types.

Currently the two syntax variants can be mixed and matched. At some point we may have some restrictions on mixing and matching. In any case semicolons will remain optional in both variants, and probably will start disappearing from examples given in future blog entries.

Both syntax variants are being supported at the moment as we are still ambivalent about whether the Python-like syntax is preferable. We have tentatively named this ParaSail-with-Python-like-syntax variant "PARython" or perhaps "Parython." We may be adding more scripting-oriented libraries so PARython might be usable in more contexts where Python is chosen today.

#### 1 comment:

1. We just updated this to revision 3.7.1. We had inadvertently left lexer debugging turned on, which meant the *.psl.lst files were loaded with debugging noise. Now they should be back to normal. The same URL listed above will get you the newer release.