Logo
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I've updated this cool sample to compile and run on latest F# release (with warnings), see post and attached code here: [link:www.trelford.com]
Logo
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Here are some samples, without any order (the last one runs a bit longer):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
// * canvas 400 500 to spiral :x repeat :x (fd 4 lt * repcount 1.1) end spiral 10000 --------------------- // * canvas 400 400 to spiral :x repeat :x (rt 90 fd * repcount 3) end spiral 100 --------------------- // * canvas 1000 1000 to polygon [:x :size] repeat :x (fd :size rt / 360.0 :x) end repeat 500 (polygon 13 20 lt 10 fd * repcount 1.01) --------------------- // * canvas 1000 1000 to polygon [:x :size] repeat :x (fd :size rt / 360.0 :x) end repeat 500 (polygon 3.5 / repcount 4 lt 10 fd * repcount 1.01) --------------------- // * canvas 1000 1000 to polygon [:x :size] repeat :x (fd :size rt / 360.0 :x) end repeat 10 (polygon + 5 / repcount 10 30 penup fd 60 pendown) --------------------- // * canvas 600 800 to spiral :x (repeat :x (fd 2 lt * repcount 2.17)) end spiral 100000 --------------------- // * canvas 1200 2500 to spiral :x (repeat :x (fd 2 lt * repcount pi)) end spiral 300000
My seven year old (and me) are having fun with this. Can you post the logo you used to make the sample images?
You can use the new list comprehension syntax to get rid of dealing with mutable data. For instance, instead of a for loop and references, you can write the Repeat match case in the eval function as
1 2 3 4 5
| Repeat (num, e, pos) -> let _, gstate', i = eval1 state gstate num in IEnumerable.fold (fun (state, gstate, _) j -> let state' = add_var state ":repcount" j in eval im state' gstate e) (state, gstate', None) { 1.0 .. i }
Note that this fixes a bug as well, and evaluating the num expression is now reflected in the graphics state. Thanks to Don for pointing this out!
Project attached - have fun!
This looks great!
Could you attach the whole F# project, so even a lazy people like me can play with it? Hm, but it looks really cool, I have to try it anyway :-).
Hi guys - I just posted an article on implementing a small Logo interpreter and produced some nice Logo...
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This first time I picked something that F# is particularly good at: implementing other programming languages. The target is Logo, your favorite turtle graphics language from grade school - mostly because it has something for everyone: it has some peculiarities from the language design angle that are worth looking at, but foremost the implementation once again shows you how easy it is to get things done with F#. So I sat down yesterday night and wrote a small Logo interpreter.
Our application has two parts: the core language module (logo.fs) that defines all types, the lexer, the parser and the evaluator; and the UI client (main.fs) that constructs the main application form and adds all interaction within.
Here is what our final application will look like (after you ran some Logo code ;)):
Logo is an interesting language because it is strikingly simple yet quite powerful and it is an excellent medium to teach kids how to program. There are a lot of implementations out there and here you will see mine without any desire to adhere to often used or even standard features - my objective is simply to be able to write short Logo programs that create nice drawings.
The language
Logo programs are made up of words; these can be commands (that do not return a value) or functions (that return words), both operating on other words. Numbers are special words. Lists of expressions are enclosed in brackets and the value of a list is the value of the last item obtained by evaluating all of list elements first. Arguments to words having a meaning are passed "by-value" and using prefix notation. Our implementation supports the following "built-in" words:
Commands and functions
Canvas e1 e2, Left (Lt) e1, Right (Rt) e1, Forward (Fd) e1, PenUp, PenDown, Repeat e1 e2, Sum (+) e1 e2, Minus (-) e1 e2, Times (*) e1 e2, Divide (/) e1 e2, Min e1 e2, Max e1 e2, Sin e1, Cos e1, Tan e1, Pi, RepCount, To
Defining new words
You can define new words (functions or commands) using To:
Single parameters can be given using a variable
The language module
First, we start by opening a few namespaces and declaring some basic types.
Here, the source type will correspond to a stream of tokens and the current position. Each token can be a word (such as "repeat"), a variable (such as ":x"), a number, or one of the bracket characters, or a "hole" (the "?" symbol). At a very basic level, everything is a word, but we attach the usual semantics to numbers and the value-of semantics to variables. The bracket characters are used to construct lists or sequences that can in turn contain other expressions. The hole symbol is actually not used in this basic implementation.
Our lexer will construct a stream of tokens that will be iterated through by the parser function, and as we consume a sequence of meaningful tokens we output an "instruction" that can be interpreted later. This instruction type is defined next:
This pretty much tells you what our Logo machine can interpret (holes are again here, but we won't work with them). We will review the concrete syntax for the instruction set shortly. Before we do that we need to define what our evaluator will work with:
The state type encapsulates the runtime environment, and stores a map of functions and variables in scope. Logo uses dynamic scoping, so a given variable or function name will refer to the "latest" binding site, so the following is completely valid and sensible:
to add :x + :x :y
This defines a new function called "add" that takes one parameter called :x, adds its value to (the value of) :y and returns this sum. We don't care that y may be unbound as it is expected that by the time we call the add function we will have a binding for y. If not, we get a runtime error.
As we evaluate our Logo programs, we will be changing the "graphics" state, e.g. the current position on the canvas, whether our pen is placed on the canvas or not, what the turtle's orientation is and what canvas we are drawing on. These are all represented in the gstate type.
Now, onto our lexer. We won't use fslex or fsyacc for this project as lexing and parsing Logo programs is unconventional. While the lexer is fairly simple, the parser itself constructs Logo instructions depending on the tokens it sees and the semantics each has. So after parsing a function definition, a call to that function will have to be parsed taking into consideration the number of arguments; e.g. our parser has a state and decides what each encountered word means and how to parse the tokens that follow.
Our lexer works with a StringReader instance passed as part of the source argument to the two core functions, eatWS and readWord, where the first skips all whitespace (spaces, newlines and tabs), and the latter consuming a word - which consists of any character other than whitespace or the brackets. Both functions keep track of the source position, incrementing on each character encountered. Newlines can only occur in whitespace, and upon finding one we start a new source line.
The actual lexer function is below:
The lexer function contructs a stream of tokens (an IEnumberable<TOKEN>), consisting of the various token shapes we discussed. Note how the last else clause determines whether a word is a number or not, returning the appropriate token. Numeric literals are just words with a special meaning.
Next is our parser. As we parse we will keep a state of variables and functions in the current scope, and to work with this state with introduce a few convenience functions and the exceptions that will be signaling the various errors eirther during parsing or running our programs.
Our parser then takes a state, a scoping predicate (discussed shortly), and the token enumerator obtained from the lexer:
Note how we construct a list of Logo instructions (last three lines) using the parse_until function defined locally. The check parameter forces a check on any variable encountered (except in formal parameters in To); this check is false by default to allow dynamic scoping, but turning it on causes lexical scoping, e.g. each variable has to be on the parameter list of one of the containing functions/commands (as this is the only means to introduce new variables in our implementation, e.g. we don't support the standard "make" Logo command).
And finally, our evaluator is a straightforward implementation of the semantics of each instruction, carefully updating the state and the graphics state:
How does the evaluator work?
During evaluation we maintain the program state and the graphics state. Each argument to a defined word is evaluated (which could affect the graphics state, so this is preserved throughout) and passed to the definition (body) of that word. Built-in words are evaluated similarly. The program state changes at three locations: after evaluating a new word definition (using To, and we add a new function to the state), during evaluation of a function/command call (we add the formal parameters to the state with the value of each argument), and during evaluating the body of a repeat command (when we add a special variable called :repcount to the state; this can be read directly or through the RepCount built-in command).
The client module
The client code is straightforward and we don't show some of the event binding to preserve compactness:
The essence of using the core Logo functionality is in the event delegate for the Run menu item. First we generate a token stream from the input string (the Text property of the edit box), parse it into our internal representation, and evaluate the resulting instruction list in the context of a child window where the results are displayed. If the evaluation yielded a value we display it after execution.
For instance, trying the following program will result in no graphics output (we get an empty child window) and the exit value of 300:
Further improvements
The implementation just described lacks a number of standard Logo features. It makes no use of Logo's higher-order functions (foreach, etc.) using holes, there are no control flow commands, etc. These can be added with a small effort and we leave that for the reader to experiment with.