TIP #194: Procedures as Values via '''apply'''
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TIP:	194
Title:	Procedures as Values via '''apply'''
Version:	$Revision: 1.6 $
Authors:	Miguel Sofer <msofer@users.sf.net>
	Joe Mistachkin <joe@mistachkin.com>
State:	Final
Type:	Project
Tcl-Version:	8.5
Vote:	Done
Created:	Fri Apr 30 00:00:00 UTC 2004
Keywords:	Tcl, lambda, anonymous, command, function, functional programming
Abstract
This TIP proposes a new command, tentatively named apply, to allow procedures to be first class values. It is an alternative to the approach of [187], where attaining a similar goal requires a syntactic and semantic change.
Rationale
Tcl is typeless, and every first class value is a string (or at least representable as such). Strings are managed by reference counting in the Tcl core, the required memory is freed automatically when the string is no longer referenced.
Tcl commands may interpret their arguments differently. Some commands interpret some of their arguments as scripts - eval, uplevel, and so on. But no current Tcl command is able to specify that a script is to be run in an isolated environment, free of unwanted side effects or memory leaks caused by name collisions.
In order to achieve this isolation, one has to define a new command via proc and assume the burden of name-collision avoidance and lifetime management, or else produce complicated scripts with ugly contorsions to avoid the name collisions.
Both [187] and this one propose ways to provide this lacking functionality to Tcl. The approach here is to do so by creating a new apply command without any change to Tcl's syntax.
Specification
A reference manual page can be found at [http://utdt.edu/~mig/apply2.html]. Summarizing, the syntax of the new command is:
apply func ?arg1 arg2 ...?
The first argument func is an anonymous function - a list of two or three elements [[list arglist body ?namespace?]]. The first two are exactly like the arguments to proc, the third determines the namespace for variable, namespace and command resolution when the body is evaluated. The namespace is interpreted as fixed, and is the interpreter's global namespace in the two-argument case.
The remaining arguments arg1 arg2 ... are taken as values for the first formal arguments in arglist.
The semantics of apply (with the exception of some of the fine details of error messages) can be described by:
 proc apply fun args 
     set len [llength $fun]
     if ($len < 2) || ($len > 3) 
         error "can't interpret $fun as anonymous function"
     
     lassign $fun argList body ns
     set name ::$ns::[getGloballyUniqueName]
     set body0 
         rename [lindex [info level 0] 0] 
     
     proc $name $argList $body0$body
     set code [catch uplevel 1 $name $args res opt]
     return -options $opt $res
 
where the availability of a getGloballyUniqueName procedure was assumed.
Reference Implementation
There is a patch [http://sf.net/tracker/?func=detail&aid=944803&group_id=10894&atid=360894] that implements this TIP.
The patch defines a new tclLambdaType for Tcl_Objs that caches the internal structures necessary for efficient evaluation: a Proc struct, a pointer to namespace, and the bytecodes implementing body. It is a small patch that relies heavily on the implementation of proc, producing essentially a regular proc with no command attached to it: an anonymous function.
All cached internal structures are freed when func ceases to be referenced or when it loses its internal representation as a tcllambdaType through shimmering.
Note that a similar approach is likely for a definitive implementation of [187].
Further Functional Programming Constructs
The availability of apply permits an easy and efficient access to other FP functions. For example one might define a constructor lambda and a curry command like this:
 proc lambda arglist body ns  
     list ::apply [list $arglist $body $ns]

 proc curry lam args 
     lappend lam expand$args
 
Function composition is also relatively easy to specify. Further examples may be seen in the Wiki, see for instance Neil Madden's map, filter, foldl, foldr [http://wiki.tcl.tk/11141] - note that the syntax is slightly different from the one proposed here.
Comparison to TIP 187 and outlook
In terms of usage, the main difference is that where TIP 187 does:
 set p [list lambda x string length $x]
 $p $foo
we would do here:
 set p [list ::apply [list x string length $x]]
 # or: set p [lambda x string length $x]
 expand$p $foo ;# or 'eval $p [list $foo]', or ...
or else:
 set p [list x string length $x]
 apply $p $foo
This TIP requires no changes to the rules in Tcl(n), whereas [187] requires a change in rule [[2]].
If in the future Tcl evolves a rule for automatic expansion of leading words, apply will provide automatically the syntax of [187].
Copyright
This document has been placed in the public domain.
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