file unsplit?
in [TCL]
|
From: Jonathan Bromley on 11 Sep 2009 14:09 On 11 Sep 2009 16:52:19 GMT, Joe English wrote: >What you can say is: "A canonical-form list, when parsed as a script, >will be interpreted as a single command whose words are the elements >of the list." That's about the only relationship between lists >and scripts I can think of... I'm not sure about "single command": % set L {puts a ; puts b} % llength $L => 5 % eval $L => a b Prodded into thinking about this more carefully than I have done before, I've learnt what most of you have always known: the parsing of a string as a *script* and the parsing of a string as a *list* are two very different things. However, they have enough similarities to confuse me (and, I suspect, others) from time to time. As Joe's intriguing example with quoted square- brackets shows, it *must* be like that for Tcl to work sensibly. Thanks to everyone for the interesting input. One day, when I try to understand [subst], I will be back for more help :-) -- Jonathan Bromley, Consultant DOULOS - Developing Design Know-how VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK jonathan.bromley(a)MYCOMPANY.com http://www.MYCOMPANY.com The contents of this message may contain personal views which are not the views of Doulos Ltd., unless specifically stated.
From: Erik Leunissen on 11 Sep 2009 16:03 Uwe Klein wrote: >>> >>> set newpath [eval [linsert $components 0 file join]] >> ... > ... > you can have it faster even: > > set newpath [if true [linsert $components 0 file join]]] > That's creative! As I understand it, both invocations cause an evaluation of the same body. So, how come that the latter is faster? Erik. > uwe -- leunissen@ nl | Merge the left part of these two lines into one, e. hccnet. | respecting a character's position in a line.
From: Uwe Klein on 11 Sep 2009 16:16 Erik Leunissen wrote: > Uwe Klein wrote: > >>>>set newpath [eval [linsert $components 0 file join]] >>> >>>... > > >>... >>you can have it faster even: >> >>set newpath [if true [linsert $components 0 file join]]] >> > > > That's creative! > > As I understand it, both invocations cause an evaluation of the same > body. So, how come that the latter is faster? > > Erik. [eval] is supposed to shimmer to string to list tokens while [if] takes the list without further ado. at least that is the explanation that was given some years ago. uwe
From: Joe English on 11 Sep 2009 23:02 Jonathan Bromley wrote: > Joe English wrote: >>What you can say is: "A canonical-form list, when parsed as a script, >>will be interpreted as a single command whose words are the elements >>of the list." That's about the only relationship between lists >>and scripts I can think of... > > I'm not sure about "single command": > > % set L {puts a ; puts b} > % llength $L > => 5 > % eval $L > => a > b Ah, but that's not a *canonical-form* list. % set L {puts a ; puts b} % lrange $L 0 end ==> "puts a {;} puts b" ............^^^ note quoting % eval [lrange $l 0 end] ==> error "wrong # args: ..." That's why the [eval [linsert ...]] idiom is recommended for callbacks: [linsert] returns a canonical-form list, so you don't need to worry about quoting Tcl syntax characters. --Joe English
From: Andreas Leitgeb on 12 Sep 2009 02:40
Uwe Klein <uwe_klein_habertwedt(a)t-online.de> wrote: >>>>>set newpath [eval [linsert $components 0 file join]] >>> you can have it faster even: >>> set newpath [if true [linsert $components 0 file join]]] >> As I understand it, both invocations cause an evaluation of the same >> body. So, how come that the latter is faster? > [eval] is supposed to shimmer to string to list tokens > while [if] takes the list without further ado. > at least that is the explanation that was given some years ago. Also some years ago there was a statement, that eval recognizes list Tcl_Obj's (internally) and avoids the string/script detour then. |