miscellany – Page 2 – The Effective Perler

keys in scalar context now returns the number of keys

Starting in v5.26, a hash in scalar context evaluates to the number of keys in the hash. You might have thought that it always did that just like an array (not a list!) in scalar context evaluates to the number of items. But nope—it evaluated to a seemingly useless number called the “hash statistics”. Now it’s fixed to do what most people thought it already did. For what it’s worth, keys (or values) in scalar context already provided the count.

Continue reading “keys in scalar context now returns the number of keys”

Apple recommends installing your own perl

Apple recommends installing your own perl (or python or ruby) for your private development to not interfere with the work the bundled perl (or python or ruby) does. In Item 110. Compile and install your own perls. we recommended the same thing. Continue reading “Apple recommends installing your own perl”

Make bitwise operators always use numeric context

[This feature is no longer experimental, starting in v5.28. Declaring use 5.28 automatically enables them.]

Most Perl operators force their context on the values. For example, the numeric addition operator, +, forces its values to be numbers. To “add” strings, you use a separate operator, the string concatenation operator, . (which looks odd at the end of a sentence).

The bitwise operators, however, look at the value to determine their context. With a lefthand value that has a numeric component, the bitwise operators do numeric things. With a lefthand value that’s a string, the bit operators become string operators. That’s certainly one of Perl’s warts, which I’ll fix at the end of this article with a new feature from v5.22. Continue reading “Make bitwise operators always use numeric context”

Use a computed label with loop controllers

Not sure which loop you want to break out of? Perl v5.18 makes that easy with computed labels. The value you give next, last, and redo no longer has to be a literal. You could already do this with goto, but now you can give the loop controllers an expression. Continue reading “Use a computed label with loop controllers”

In v5.20, -F implies -a implies -n

Perl was once known for its one-liners in its sysadmin heydays. People would pass around lists of these one liners, many of which replaced complicated pipelines that glued together various unix utilities to do some impressive system maintenance. Continue reading “In v5.20, -F implies -a implies -n”

Declare packages outside of their block

Perl v5.14 gets a step closer to a saner way to declare classes with its new package NAME BLOCK syntax that lets you easily group everything that goes in a package. Continue reading “Declare packages outside of their block”

The Perl 5.12 yada yada operator

Perl v5.12 adds a placeholder operator, ..., called the yada yada operator, after an episode of Seinfeld where the interesting parts of the story are replaced with “yada yada yada”. Continue reading “The Perl 5.12 yada yada operator”

Create your own dualvars

Perl’s basic data type is the scalar, which takes its name from the mathematical term for “single item”. However, the scalar is really two things. You probably know that a scalar can be either a number or a string, or a number that looks the same as its string, or a string that can be a number. What you probably don’t know is that a scalar can be two separate and unrelated values at the same time, making it a dualvar. Continue reading “Create your own dualvars”

Turn off autovivification when you don’t want it

Autovivification, although a great feature, might bite you when you don’t expect it. I explained this feature in Understand autovivification, but I didn’t tell you that there’s a way to control it and even turn it off completely. Continue reading “Turn off autovivification when you don’t want it”

Set the line number and filename of string evals

Errors from a string eval can be tricky to track down since perl doesn’t tell you where the eval was. It treats each of the string evals as a separate, virtual file because it doesn’t remember where the string argument came from. Since perl compiles that during the run phase (see Know the phases of a Perl program’s execution), the information the compiler dragged along for filenames and line numbers is so longer around. Continue reading “Set the line number and filename of string evals”