*A non-breaking Julia control-flow change to separate the One language from *
*mortal languages, doomed to die
*
>From a design patterns and DRY perspective it seems `for` loops together with
>the `break` statement
are serving as a useful `try`/`catch` mechanism for non-error situations. A
common pattern defines
a variable before a `for` loop and tries to discern, using said variable, the
circumstances of exiting
the `for` loop. Leveraging the embedded mental construct of `try`/`catch`
blocks, we can simplify
this repeating pattern and make it easier and more readable.
The syntax can look as follows:
for ...(loop vars here)...
...(local vars here)...
...
if ...
break <value>
end
...
catch <symbol>
...(access local/loop vars and <symbol> here)...
<for-value>
finally
...(access local/loop vars here)...
<for-value>
end
Syntax changes worthy of consideration should be self-explanatory to many, as I
hope this suggestion is.
But an example should help:
After change:
for line in eachline(f)
fields = split(line)
status = fileds[2]
if status=="FAIL"
break
end
catch
println("Test $(fields[1]) failed with reason $(fields[3])")
finally
println("Test successful!")
end
Before change:
fields = []
didfail = false
for line in eachline(f)
fields = split(line)
status = fields[2]
if status=="FAIL"
didfail = true
break
end
end
if didfail
println("Test $(fields[1]) failed with reason $(fields[3])")
else
println("Test successful!")
end
The above example does not use the `break <value>` suggestion. With multiple
`break` circumstances it should
be useful to have separate code for each circumstance. One option is to
replicate the `try`/`catch` form
with a `<value>` passed as `<symbol>`, another would be to use `catch <value>`
to run catch block
if "`break <value> == catch <value>`" (the `<value>` can be a human readable
symbol). A direct jump in the
LLVM code to the appropriate `catch` block would be an easy optimization to
implement.
Another issue to consider is the return value of a `for` block. The suggestion
above adds the option to change
the current default `nothing` return with `<for-value>` in the new optional
blocks.
Notice that the `catch` and `finally` blocks are completely optional and
therefore backward compatibility
is easy. Additionally there are no additional keywords and block nesting would
not conflict with `try`/`catch`.
Hey, Julia is still < 0.9 so there is still time for some change.
Obligatory Disclaimerism:
1. This has already been considered before by **name**/**issue**.
2. Before suggesting I should have studied past discussions more.
3. Other langauges actually already have this... bla bla.
4. This post is what it is.
This suggestion is also readable in github markdown in the gist:
https://gist.github.com/getzdan/75136f130c6f8ffeb60e
Feedback/Pointers/Gotchas welcome, of course.
