perlmeditation
Juerd
<!-- *******************
Note to janitors: I used a pre tag because the diagram gets messed up with line wrapping. Please don't change it to a code tag.
******************* -->
<pre><tt>
This is a simplified high level view of relations between Perl scalar types.
┌───────────────┐
┌───── │ REFERENCE │ ─────┐
│ │ (ROK flag on) │ │
│ └───────────────┘ │
numeric context │ │ string context
│ │
▼ ▼
┌─────────────────────────┐ ┌──────────────────────────────┐
│ NUMBER │ string context │ TEXT STRING │
│ encoded internally │ ──────────────────▶ │ (POK flag on) │
│ as any of: │ │ encoded internally │
│ * integer (IOK flag on) │ numeric context │ as one of: │
│ * double (NOK flag on) │ ◀────────────────── │ * iso-8859-1 (UTF8 flag off) │
└─────────────────────────┘ │ * utf8 (UTF8 flag on) │
│ ▲ └──────────────────────────────┘
│ │ ▲ │ ▲
│ │ │ │ │
pack │ │ unpack decode │ │ encode │
│ │ │ │ │
▼ │ │ ▼ │ :encoding
┌─────────────────────────────────────────┐ │ PerlIO
│ BINARY STRING │ │ layer
│ (POK flag on) │ ◀───────┘
│ (UTF8 flag off) │
└─────────────────────────────────────────┘
▲ ▲ ▲
│ │ │
│ │ │
▼ ▼ ▼
┌─────────────────────────────┐
│ OUTSIDE PERL │
│ files, sockets, filenames, │
│ environment, system calls │
└─────────────────────────────┘
(A Perl programmer does not have to know about the internal flags ROK, IOK,
NOK, POK, and UTF8, but if you're interested read perlguts.)
Keep text and binary strings/semantics separated! (Good style anyway!)
If you don't keep them separate, and use a binary string as a text string, it
is assumed to be iso-8859-1 encoded.
If you don't keep them separate, and use a text string as a binary string, one
of the following things happens, with or without warnings:
1. the internal iso-8859-1 buffer is used (always the case if the internal
buffer is not utf8 encoded)
2. the internal utf8 buffer is used
3. the iso-8859-1 encoded version is used
3a. characters above U+00FF are utf8 encoded, while the rest is iso
3b. characters above U+00FF are modulo'ed 256
3c. characters above U+00FF are dropped
3d. characters above U+00FF cause an exception to be thrown
If you do keep them separate, and always explicitly convert between the two
types by explicitly decoding and encoding or using the :encoding layer on a
filehandle, you stay in control of what happens and your program will behave
more predictably.
</font></tt></pre>
Update: thin lines used, see discussion below.