acos (arc cosine) is one of the math commands available in [expr], as well
as in the tcl::mathfunc:: namespace as of Tcl 8.5.

It is a procedure which provides command access to the [expr] math function, taking the same arguments as the expr function and taking expressions as arguments as well.

[LV] 2007 Oct 31

Is the following one of the differences between using acos in expr and as a
command? The above comment says "taking the same arguments as the expr function 
and taking expressions as arguments as well" - is there something special
needed for the expressions to the procedure?

 $ tclsh8.5
 % expr {acos(4-5)}
 3.141592653589793
 % tcl::mathfunc::acos {4-5}
 expected floating-point number but got "4-5"

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One good use for [acos] is to get the constant Pi:

 % expr acos(-1)
 3.141592653589793
 % tcl::mathfunc::acos -1    ;# In Tcl 8.5
 3.141592653589793

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%| [Category Command] | [Category Mathematics] | [Category Syntax] |%