Example: An existence map of ZIP codes between 00000 and 99999 can be kept in a list of 3125 integers (where each element requires about 15 bytes overall), while implementing the map as an array would take 100000 * 42 bytes in worst case, but still more than a bit vector if the population isn't extremely sparse in that case, a list of 1-bit positions, retrieved with lsearch, might be more efficient in memory usage. A very simple control structure (one might also call it a result dispatcher) is the K combinator, which is almost terribly simple: It can be used in all situations where you want to deliver a result that is not the last. Filters may be characterized as "selectors" (who may return only part of their input, like "grep") and/or "appliers" who call a command on their input and return the result. His two axioms are: and these can be implemented by a string map that is repeated as long as it makes any difference (sort of a trampoline) to simplify any expression consisting only of operators and constants (which are operators with zero arguments): which maps <><> to <>, <<>> to "", and returns <> for "true". Completing it unlocks the rest of the Tcl Track. with our unique blend of learning, practice and mentoring. The Tcl Programming Language is a comprehensive guide to Tcl, covering Tcl 8.6.. See the official book page for more information and a detailed Table of Contents.. This means that subsequent calls to know stack up, last condition being tried first, so if you have several conditions that fire on the same input, let them be "known" from generic to specific. 122 exercises # - rewrite a proc's default arg with given value. Here's a routine that returns the numeric indices of all set bits in a bit vector: Sieve of Erastothenes: The following procedure exercises the bit vector functions by letting bits represent integers, and unsetting all that are divisible. In a nutshell, his FP system comprises. Also, memory limits on modern computers are somewhere up high so only at some time in the future you might have (but maybe not want) to change to a complex database;-). The following example code opens a file, configures it to binary translation (i.e. So here is one model of a state machine in ten lines of code. Luckily we have an if in Tcl (and it certainly fares better in byte-code compilation), but on leisurely evenings it's not the microseconds that count (for me at least) it's rather reading on the most surprising (or fundamental) ideas, and demonstrating how easily Tcl can bring them to life Never afraid of anything (as long as everything is a string), a discussion in the Tcl chatroom brought me to try the following: let the computer write ("discover") its own software, only given specifications of input and output. However, it fails to work if we add the successor of 0 as another test case: Nothing coming because zero division made the last test fail. # This simple but infinite stream source produces all positive integers: # This produces all (well, very many) powers of 2: # A filter that reads and displays a stream until user stops it: # Here is a sample usage with famous name: #. which uses the (less) famous function maker: # Usage example: more {grep this {cat streams.tcl}}. Running a Tcl/Tk applet within a Tcl/Tk program. was instigated by the fact that in J, "NB." I won't go into all details of the above code, just some: (<.,>.) In his Turing Award lecture, Can Programming Be Liberated from the von Neumann Style? which is shorter and simpler, but meddles more directly with the stack. Just choose how to implement instance variables: The task of frameworks, be they written in Tcl or C, is just to hide away gorey details of the implementation in other words, sugar it:) On the other hand, one understands a clockwork best when it's outside the clock, and all parts are visible so to get a good understanding of OO, it might be most instructive to look at a simple implementation. Rather, one could use read and write traces on variable M, causing it to load from, or store to, mem($HL). In this article, we will know how to use procedures in TCL. The authors provide sample chapters available to download for free. But this version, that maps double first, works: One more experiment, just to get the feel: which gives 5.0. Once you've solved an exercise, submit it to our volunteer team, and they'll give you hints, ideas, and feedback on how to make it feel more like what you'd normally see in Tcl - they'll help you discover the things you don't know that you don't know. The language is commonly used for rapid prototyping, scripted applications, GUIs, and testing. Tcl was designed for creating domain-specific languages. This code for transposing a matrix uses the fact that variable names can be any string, including those that look like integers, so the column contents are collected into variables named 0 1 2 and finally turned into the result list: An integer range generator produces the variable names, e.g iota 3 => {0 1 2}. A further optimization could be to tally value strings, and replace the frequent ones with "@$id", where db(@$id) holds the value once, and only db'get has to be adapted to redirect the query. I'm far from having digested it all, but like so often, interesting reading prompts me to do Tcl experiments, especially on weekends. The set of all defined bytecodes will be the alphabet of this little RPN language. They are however better reusable than the multable proc above. ): proc flatten_list { l } { if { [llength $l] == 0 } { return {} } elseif { [llength $l] == 1 && [lindex $l 0] == $l } { return $l } else { set ret {} In both cond and body you may use the variable args that holds the problem command unknown was invoked with. Clif first learned to program in high school in machine language on a Monroe 600 programmable . The memory model is constant-size instructions (strings in array elements), which are implemented as Tcl procs. The "main routine" is a single line that dumps all files given on the command line: Sample output, the script applied to itself: Roman numerals are an additive (and partially subtractive) system with the following letter values: Here's some Tcl routines for dealing with Roman numerals. All Tcl files will have an extension, i.e., .tcl. Bertrand Russell commented that the author "has revealed a new calculus, of great power and simplicity" (somehow sounds like Tcl;^). So what about a thin abstraction (wrapper) around this recurring pattern? Here, pushing has to be done by dedicated code because a previous instance would have to be removed: The first element is the least recently, the last the most recently used. So, on a morning dogwalk, I thought out this strategy: Here's this version. Bit vectors can also be used to indicate set membership (set operations would run faster if processing 32 bits on one go with bitwise operators (&, |, ~, ^)) or pixels in a binary imary image, where each row could be implemented by a bitvector. bit foo 32 will turn foo into a list of two integers, if it was only one before. In other words, a tautology. In J, it looks like this: which may better explain why I wouldn't want to code in J:^) J has ASCIIfied the zoo of APL strange character operators, at the cost of using braces and brackets as operators too, without regard for balancing, and extending them with dots and colons, so e.g. Implement an evaluator for a very simple subset of Forth. To try this in Tcl, here's a truth table generator that I borrowed from a little proving engine, but without the lsort used there the order of cases delivered makes best sense when the first bit is least significant: }. Accessing fields in a table is more fun with the field names than the numeric indexes, which is made easy by the fact that the field names are in the first row: Here is how to filter a table by giving pairs of field name and glob-style expression in addition to the header line, all rows that satisfy at least one of those come through (you can force AND behavior by just nesting such calls): This filters (and, if wanted, rearranges) columns, sort of what is called a "view": In the following few chapters you'll see how easy it is to emulate or explore other programming languages with Tcl. if someone mentions its name (minibot), tries to parse the message and answer. Tcl is a scripting language somewhat like Perl but extensible and clearer. (after 286 seconds): After partitioning, 54005 programs had the -1 stack balance, and the correct result was on position 48393 in that list And finally, with the half-million set of programs, here's a solution for the successor function too: "d-" subtracts top of stack from itself, pushing 0; the second duplicate to the 0-th power gives 1, which is added to the original argument. $ wish ex1proc.tcl. The source files for Tcl programs are named with the extension ".tcl". Maybe another weekend John Backus turned 80 these days. However, this is no fundamental problem consider that. But this very soon crosses the limits of integers, giving wrong results. Here we can do what we want, even retrieve which fields we have used so far (using a temporary array to keep track of field names): Searching for records that meet a certain condition can be done sequentially. If both the operands are non-zero, then condition becomes true. which, when called with no arguments, return 1 or 0, respectively. One could edit that file (not recommended), or rename unknown to something else and provide one's own unknown handler, that falls through to the original proc if unsuccessful, as shown in Radical language modification. Hence, streams can be (and typically are) nested for processing purposes. On the limits: Tcl arrays may get quite large (one app was reported to store 800000 keys in Greek characters), and at some point enumerating all keys with array names db (which produces one long list) may exceed your available memory, causing the process to swap. They can be more precise than any "float" or "double" numbers on computers, as those can't exactly represent any fractions whose denominator isn't a power of 2 consider 13 which can not at any precision be exactly represented as floating-point number to base 2, nor as decimal fraction (base 10), even if bignum. Indexes: As shown, we can retrieve all data by sequential searching over array names. Conversely, postulating non-equivalence turns out to be false in all cases, hence a contradiction: So again, we have a little proving engine, and simpler than last time. Clif is the author of the TclTutor package2 and the books Tcl/Tk for Real Programmers and Tcl/Tk: A Developer's Guide3. This way, a stream can promise to be a potentially endless source of data, while taking only finite time to process and deliver what's really wanted. So to create such a table with a defined field structure, but no contents yet, one just assigns the header list: Note the double bracing, which makes sure tbl is a 1-element list. As a second step, we create the If command that wraps the expr invocation: This again passes impromptu tests, and adds the feature that any non-zero value counts as true and returns 1 if we neglect the other syntactic options of if, especially the elseif chaining. # now do something with db($key) - but see below! The extending code what is prepended to the previous unknown body. (I might have called it fun as well it sure is.) Every language has its own way of doing things. No con-/destructors are needed, in contrast to the heavierweight matrix in Tcllib. A range (numeric or strings) can be given as from..to, and the associated scriptlet gets executed if the tested value lies inside that range. Tcl (Tool Command Language) is a very powerful but easy to learn dynamic programming language, suitable for a very wide range of uses, including web and desktop applications, networking, administration, testing and many more. The print version is available from Amazon in USA/Canada and their European sites including UK , Germany , France , Spain and Italy . Live Demo #!/usr/bin/tclsh puts "Hello, World!" Assuming, Tcl environment is setup correctly; let's run the program after switching to file's directory and then execute the program using $ tclsh test.tcl I know there are many table implementations in Tcl, but like so often I wanted to build one "with my bare hands" and as simple as possible. Procedures in Tcl cover what other languages call procedures, subroutines, or functions. They are for instance the building blocks of relational databases and spreadsheets. Retrieving fields that may not physically exist needs a tolerant access function: In a classical database we have to define tables: which fields of what type and of which width. Introduction to the Tcl 3 Language The next ve chapters constitute a Tcl language tutorial. # make a list of 2**n lists, each with n truth values 0|1, #-- And here's some more hand-crafted bytecode implementations, #-- The stack routines imply a global stack::S, for simplicity, # a bytecode will consume at most two elements, #-- make a table of bytecode stack balances, #-- "peephole optimizer" - suppress code with redundancies. What's missing is the capability to randomly address parts of a stream, as is possible in Scheme (and of course their claim to do without assignment, or mutable data) Tcl lists just don't follow LISP's CAR/CDR model (though KBK demonstrated in Tcl and LISP that this structure can be emulated, also with procs), but rather C's flat *TclObject[] style. The other words (arguments) are not substituted because they're curly-braced, so either 0 or 1 is invoked, and does its simple job. 122 exercises to help you write better code. But for historical reasons, the Tcl command to create a function is called proc and thus people most often call them procedures. Single bytecodes are executed, only to measure their effect on the stack. A filter takes one or more streams, and possibly other arguments, and reacts like a stream too. They always return a result (even if it is the empty string ""), so to call them functions might be most appropriate. Any proc must however be called in compliance with Tcl's fundamental syntax: first word is the command name, then the arguments separated by whitespace. It augments the current unknown code, at the top, with a handler for. # That's it. Note that as my lmap above only takes one list, the two-list case had to be made explicit with foreach. In Tcl, a sensible implementation for compact data storage would be as a list of lists. # Multiple documentation lines are allowed. Ramanujan numbers: The pairs generator can be used to find Ramanujan numbers, which can be represented as the sum of two integer cubes in more than one way. Just say "Hello, World!". For Joy's sets I haven't bothered yet they are restricted to the domain 0..31, probably implemented with bits in a 32-bit word. Implementation is as a "little-endian" list of integers, where bits 0..31 are in the first list element, 32..63 in the second, etc. true. The coin values should be passed to change as a variable number of arguments which are the coin values in units (e.g., a quarter would be represented as 25) in any order. all The goto command is defined "locally", and deleted after leaving the state machine it is not meaningfully used outside of it. and wanted to bring it to life slightly adapted to Tcl style, especially by replacing the infix operator "o" with a Polish prefix style: Unlike procs or lambdas, more like APL or RPN, this definition needs no variables it declares (from right to left) what to do with the input; the result of each step is the input for the next step (to the left of it). Threads are for people who can't program state machines.". My "Def" creates an interp alias, which is a good and simple Tcl way to compose partial scripts (the definition, here) with one or more arguments, also known as "currying": The second parameter, "=", is for better looks only and evidently never used. So [or] == 0 and [and] == 1. Streams are interesting if they don't deliver the same result on every call, which requires them to maintain state between calls e.g. If a filter meets end-of-stream, it should return that too. In TOOT, the values of objects are represented as a list of length 3: the class name (so much for "runtime type information":-), a "|" as separator and indicator, and the values of the object, e.g. For instance, here's a breathtakingly short J program to compute the mean of a list of numbers: Only implicitly present is a powerful function combinator called "fork". Mathematically put. For creating FORTRAN and the BNF style of language description, he received the ACM Turing Award in 1977. This simple example invokes expr if the "command" is digestible for it: Imagine the makers of Tcl had failed to provide the if command. #-- Move tape Left or Right as specified in rule. . Note that on infinite streams, selectors may never return, e.g. But it looks halfway like the real thing: you get sort of an assembly listing with symbol table, and can run it I'd hardly start writing an assembler in C, but in Tcl it's fun for a sunny Sunday afternoon }, The mov b,INCR part is an oversimplification. TCL scripting is much sought after skill set for every VLSI engineer. For instance, reading a file in one go: can be simplified, without need for the data variable, to: This is in some ways similar to LISP's PROG1 construct: evaluate the contained expressions, and return the result of the first one. is building a list of the floor and the ceiling of its single argument, the comma being the concatenation operator here, comparable to Backus' "construction" or Joy's cleave. And when both x and !x exist, they are removed from the expression: translated back: "I avoid it, or it's not a kangaroo", or, reordered, " a" which by (4) means, "All kangaroos are avoided by me". Get better at programming through fun, rewarding coding exercises that test your understanding of concepts with Exercism. Especially constants (like "true" and "false" in Boolean algebras) can be equally well expressed as neutral elements of operators, if these are considered variadic, and having zero arguments. There are over 200 exercises with solutions for both Unix and Windows platforms. Other streams may provide a finite but very large number of elements, which would be impractical to process in one go. 1. foreach loop Use: Where we have to iterate on each element on a list of elements and have to perform some operation on each element. Saving also goes a good way to what is ceremonially called "committing" (you'll need write-locking for multi-user systems), while loading (without saving before) might be called a "one-level rollback", where you want to discard your latest changes. The test suite at end should give many examples of what one can do in "r". 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