Editing Eval (section)

=== bs ===
[[bs (programming language)|bs]] has an <code>eval</code> function that takes one string argument. The function is both an expression evaluator and a statement executor. In the latter role, it can also be used for error handling. The following examples and text are from the <code>bs</code> [[man page]] as appears in the [[UNIX System V]] Release 3.2 Programmer's Manual.<ref name=SVR32>{{cite book
  |title = UNIX Programmer's Manual
  |section = Volume 1 Commands and Utilities
  |year = 1986
  |publisher = AT&T
  |page = 41
  |url=https://bitsavers.trailing-edge.com/pdf/att/unix/System_V_Release_2/UNIX_Programmers_Manual_Vol_1_Commands_and_Utilities_1986.pdf}}
</ref>

{{Quote
|text=The string argument is evaluated as a <code>bs</code> expression. The function is handy for converting numeric strings to numeric internal form. The <code>eval</code> can also be used as a crude form of indirection, as in the following (Note that, in <code>bs</code>, <code>_</code> (underscore) is the concatenation operator.):
<syntaxhighlight lang="bash">
name = "xyz"
eval("++" _ name)
</syntaxhighlight>
which increments the variable <code>xyz</code>.

In addition, <code>eval</code> preceded by the interrogation operator, <code>?</code>, permits the user to control <code>bs</code> error conditions. For example:
<syntaxhighlight lang="bash">
?eval("open(\"X\", \"XXX\", \"r\")")
</syntaxhighlight>
returns the value zero if there is no file named "XXX" (instead
of halting the user's program). 

The following executes a <code>goto</code> to the label <code>L</code> (if it exists):
<syntaxhighlight lang="bash">
label = "L"
if !(?eval("goto " _ label)) puterr = "no label"
</syntaxhighlight>
}}