'''[http://www.schemers.org/%|%Scheme]''' is derivate of [Lisp]. A programming
language with even less syntax than Tcl itself. (but special forms and the
like...? [RS])
''Scheme is a lexically-scoped, block structured, dynamically typed, mostly functional language. It is a variant of Lisp. It has first-class procedures with block structure and indefinite extent. Parameter passing is by value, but all values are references. It has first-class continuations to allow the construction of new control abstractions. It has lexically-scoped ("hygienic") macros to allow definition of of new syntactic forms, or redefinition of old ones.'': [ftp://ftp.cs.utexas.edu/pub/garbage/cs345/schintro-v14/schintro_toc.html%|%An Introduction to Scheme and its Implementation]
** See Also **
[Biglook]:
[Muddy Scheme]: a Scheme implementation in [Tcl]
[Playing Scheme]:
[Tcl and LISP]:
[Playing LISP]:
[Serial summing]:
[Cameron Laird%|%Cameron Laird's] [http://phaseit.net/claird/comp.lang.scheme/scheme.html%|%personal notes on Scheme]:
[tcl-scheme], by Ian Bicking : a translation of Tcl into [Guile]/[Scheme]
** Reference **
[BOOK Structure and Interpretation of Computer Programs%|%Structure and Interpretation of Computer programs], by Harold Abelson, Gerald Jay Sussman, Julie Sussman:
[http://okmij.org/ftp/Scheme/monad-in-Scheme.html%|%Monadic Programming in Scheme]:
** Tools **
[PS/Tk]: Portable Scheme interface to [Tk]
[Chicken/Tk], by [wdb%|%Wolf-Dieter Busch]: An interface to Tk, based on [scheme_wish]
[scheme_wish], by Sven Hartrumpf: a portable interface between Scheme and Tcl/Tk
[Scheme Tk]: a free R4RS Scheme interpreter that can access Tk. Concretely, it can be seen as the standard Tk package where Tcl has been replaced by a Scheme interpreter
[Scheme Widget Library]: a graphics package for [http://www.scheme.com/%|%Chez Scheme], based on [Tk]
** Description **
[wdb]: I strongly disagree that Scheme has less syntax than Tcl! Reason: the symbols have a value ''per se''. Tcl has no symbols as $varname is just a shortcut for [[set varname]] where ''varname'' is just a constant, and the procedure addressed by ''set'' carries all the semantics. In Tcl, the command ''list unknown'' returns ''unknown'' because it is a constant. In Scheme, ''(list unknown)'' throws an error unless a symbol ''unknown'' is defined. If you want to list an unknown symbol, indeed, you need the special form ''(quote unknown)'' resp. its shortcut '' 'unknown''.
* In Lisp, the special form ''quote'' is essential.
* In Tcl, it is not necessary as it can be defined as a regular procedure as follows:
======
proc quote x {
set x
}
======
(Thank you, [RS], for the hint with the combinator ''[identity function%|%Identity]''!)
----
One interesting difference is that setting of (only global?) variables and function definition both are done with ''define'', which creates a lambda if its first element is a list. Just for experimenting, here's how to have that in Tcl:
======
proc define {what how} {
if {[llength $what] == 1} {
uplevel 1 set $what [list $how]
} else {
proc [lindex $what 0] [lrange $what 1 end] $how
}
}
======
======none
% define x 42
42
% define {+ x y} {expr {$x+$y}}
% + $x $x
84
======
This prevents us from having spaces in variable names (not a big problem), but presents the function name and arguments in one list, similar to how it will later be called - people sometimes wonder why we define `proc foo {x y} {..}` but call it `foo $x $y`...
----
[silas] 2018-11-14: LISP has the quote operator ('), as described above, but also has the backquote operator (`) that does what (in Scheme parlance) is called quasi-quotation. It will not eval anything unless lists followed by commas. Take the following Scheme code as an example:
======none
(define (nl) (display "\n"))
(define x 10)
(define v '(display x))
(display v)
(nl)
(eval v)
(nl)
(define a 20)
(define y `(display ,(+ a 10)))
(display y)
(nl)
(eval y)
(nl)
; Output is:
; (display x)
; 10
; (display 30)
; 30
======
In Tcl I've stumbled upon situations where I needed something similar. Usually people use `eval "..."` and escape `\$` and `\[` when don't want it to be evaluated, but it can lead to more or less unreadable code. I wrote a small method called xset that works just like [set], but replaces variables that start with `@`:
======
proc xset {var body} {
# Look for own @ identifiers in $body
set my_identifiers [regexp -inline -all {@[[:alpha:]]+} $body]
set my_identifiers [lsort -unique $my_identifiers]
set map_list [list]
# Builds the map_list variable
foreach my_ident $my_identifiers {
set ident [string map [list "@" ""] $my_ident]
upvar 1 $ident x
# If upvar'ed variable does not exist, raise an error.
if {[catch {lappend map_list $my_ident $x}]} {
set raw_name [string map [list "@" ""] $my_ident]
return -code error \
-errorinfo "can't read $raw_name: no such variable"
}
}
# Make substitutions, set and return $body.
upvar 1 $var x
set body [string map $map_list $body]
set x $body
return $body
}
======
And example to that would be:
======
set one 1
xset test {
set one "test"
set a $one
set b @one
}
puts $test
# outputs:
# set one "test"
# set a $one
# set b 1
======
How would you implement quasi-quotation in Tcl?
<<categories>> Language | Functional Programming