In functional and semi-functional languages including Scheme, Javascript, Haskell, ML , functions are '''First Class Citizens , meaning that , like any other value , they can be passed as arguments to other functions . By definition, they do not exist in languages such as C , Perl , and Algol , where functions inhabit a different namespace than variables. As with many things , Tcl is rather unique in this area . commands themselves can not be passed as arguments to other commands , but their names can , which provides something very similar to first-class functions . Additionally , apply can be used to interpret a value as a function and evaluate it , providing the equivalent of lambda functions .
As a "fisrt-class citizen" , a function can be
like any other value . In other langauges , functions are usually opaque objects, so introspection is not available . In Tcl , they are simply strings
First-class functions permit coding styles that are not available in languages without them (C is a good example):