Older blog entries for tampe (starting at number 116)

One year of imagination and coding

This year I have accomplish much more than any year before, Maybe not so much is seen in the space of Openly sourced code, but quite a lot have been achieved under the hood. For example I have been studying the guile sources extensively by experimenting compiling guile scheme to native code, and making an rtl compiler. I'm pretty new here and actually not trusted to make such a pillar of a component of guile. I'm pretty fine with that. I really enjoy the programming effort and will of cause use this experience in the open discussions how to churn out these things in the end when the trusted hackers starts hacking. Another reason I do not want to promote my code is because it's learning code, e.g. I'm not that good and experienced programmer to churn these things out to the best degree. The native code works, can be 2-3 times faster then rtl that maybe is 50% than guile-2.0 and it can be made working. But it's not the best you can do!

My approach is simple. Just take a byte code and inline a C-stub that represents that code. Many operations would be just inlining essentially a function call out to a C - stub that does the meaty work. The rest, could be efficiently coded with a few cpu instructions. The fear with this approach is that we will end up with a bloated code that blows the instruction cache. I'm not sure about this, I just observed the number of bytes to encode a simple function was less that encoding it in actual byte code so I do not buy this yet.

Nah the main problem is that I don't take advantage of cpu registers. For example there are JIT engines out there that does register handling so it is important in order to get up in speed. But I wan't the engine to be coded in scheme - which maybe is stupid, but after coding quite a lot of assembler in scheme I'm convinced that getting that functionality into scheme is a boon.

Another big task I have been taken is to make sure guile can compile scheme down to the rtl VM that wingo has coded. It was impossible for me to do that on a clean piece of paper so I took the old compiler to the old VM and molded it into produce RTL VM code instead. Cool, but remember we need to make use of registers! And the rtl vm representation as the old guile VM representation is not optimal to use for finding out the allocations for the registers. We need something new and Noha has been starting coding on such a compilation scheme. But Now I now all the details needed to produce nice rtl vm code - so I can be a good help in his quest.

Happy Hacking :-)

I order you to try the next calculation

Here is the problem I generate sequences of elements in parallel but they don't come up in sync some elements are dropped and some are added on one, both or the other.

Well this can be solved by plain old generators. But I want the backtracking features of prolog at my fingertips in order to generate this - it's nice, and I also would like to, at will, save the state and later redo the calculation with perhaps some other settings of some global variables

So again, the kanren way to handle this can be done, but if one want to put everything in tail call position and not use the common trick of sending info by returning the function the normal way I really don't know how to code this. With kanren you can use variables with values on the heap and set! them to update information, this can work, but I would also like to store and retrieve the state and then letg variables are ideal to use. So I coded a /> ideom using this and this is how it look

(%||% ((next-1 ((x xx)) (f 0.1 x))
(next-2 ((x yy)) (f 0.2 x)))

;;Guard phase
(if (condition-1 xx yy) (%update% next-1))
(if (condition-2 xx yy) (%update% next-2))

body-of-the-rest ...)

'%||%' has the model

(/> ((Next ((x xx) ...) code ...) ...)
body ...)))

this idiom will generate in parallel values of x that is copied over to xx and yy, then depending on the values either xx or yy is updated if they are out of sync, if they are in sync then the body will be evaluated and finally when the body backtracks both next-1 and next-2 will be updated. Nice It does what I want. I can also use (%update% (next-1 failure)) if failure is a a particular point inside
the code in next-1. I keep it in a form that allow me to use the implementation in languages without call/cc like constructs, it uses almost no stack space if proper tail-call's are implemented. It properly and magically manages state so that it can be retrieved or in case where the state is not stored appropriately unlink the overhead so it can be GC:ed. If speed is needed well, then consider using generators or the let version of the code. For the ones that thinks that functional code is the golden egg remember that I do employ functional techniques, for example in the unique function that I implemented in the previous post, the list could be replaced with a functional tree in order to improve scalability. Cool In order to be non-functional I need to be functional (partly)

Illustrations of logic programming ideoms

Disclaimer. I'm self educated when it comes to logic programming so I might be out on the water a bit, still I prefere to have a nice cool bath then just making sand castles

Ok, step 1 in this discussion is to note a need. I would like to try a logical branch and if it succeeds I want to use the result without the cruft needed to be used to calculate the result. E.g. store the result and backtrack to the beginning. Now if I later conclude that the results was not useful I would like to backtrack and redo the logical branch, back track that branch and calculate a new value. - sounds complex doesn't it.

Why would you like to do this. 1 in the process of calculating a value some other variables gets a binding as a result you would like to undo those settings and only use the conclusion. Also forcing this kind of call to only succeed once means that you can save stack storage.

Let's go to the meat,

(define (call s p cc lam x l)
(use-logical s)
(let ((s (gp-newframe s)))
((gp-lookup Lam s)
s p (lambda (ss pp)
(let ((state (gp-store-state ss)))
(let ((xx (gp-cp x)))
(gp-unwind s)
(let ((ppp (lambda ()
(gp-restore-wind state)
(leave-logical s)
(%with-guile-log% (s ppp cc)
(%=% xx l)))))))))

This shows how this call mechanism is coded in guile-log.
use-logical means that redoing state is fast e.g. it's using the kanren approach to bind values with a translation table in the form of an assoc list, also variables is allocated on the heap. The drawback is that unification can be slow and if there is a lot of variable bindings going on then the assoc can be huge and we can get inferior performance. Because the only needed feature is the allocation of the variables on the heap, there is plenty of options to target this function towards different use-cases. note the argument list. All functions have the first three, s = state and assoc list, p is the failure and cc is the continuation. lam is the logic x is the result probe and l is the output variable where the result (x) should be unified to. As we continue in the function we make a newframe e.q. a point to backtrack to. Lam is a lambda e.g. a function with only the s p and cc argument here we code a special cc. It basically set the state after the calculation of the logic in Lam in the local variable state, then copy the result probe x to xx unwind to the beginning, leave the logical setup and unify xx with the output l and we go to the continuation! Note here how the ppp e.g. the failure lambda send to the continuation just restore the state and issue a backtrack

gp-cp is mapping x by looking up values and put them in the list or leave a variable at the place. Because variables are allocated from the heap they will be ok to use after all links have been backtracked and not be overrun when the continuation kicks in. Note here a quite huge issue. Assume that you have a variable that was alive at the beginning and inside the Lam you do a unification with a newly created variable, as it now is, what points to what is random and no rule is used so the function above is not really sound at the moment you can get a hook onto an old variable or not randomly. What you would like to do is to have a creation number on the variable and when you unify two variables you take the convention to point the newer to the older if not stated directly via a gp-set a form. But lets think away this problem for now and continue to the next step

For this step I would like to take a logic Lam, then a probe X and an output L then I would like to collect all possible answers X in the list L and L should be the result. Here is the code in guile-log

(%define% (collect Lam X L)
(%letg% ((l '()))
(%funcall% Lam)
(%code% (set! l (cons (gp-cp X S) l)))
(%=% l L)))))

I use % in stead of guile-log's markers and work in a small sub language where I do not keep track of s p cc all the time plus some other feature. So again with logical++ we make sure to use variables on the heap. We create a letg variable l, that will contain the list we build up. the g stands for ... ! well i don't remember. but the semantic is that with it. you can store and retrieve states at will e.g. stop a calculation do something store the state run it again, at another point in time restore the state and run it another time perhaps with some other parameters. It also makes the algorithm slower so if one needs the speed just nuke the g in letg. Anyway we call the logic inside Lam and again make a copy of the probe X and cons it onto the list. When there is no more solutions left the next field in the or is run turning of the kanren (logic--) feature if it was not on from the beginning and unify the the result list to L. You might want to do a reverse of the list here as well.

For step 3 we mold the evaluation by forcing a probe to vary form result to result. We try this with perhaps,

(%define% (%uniq% Lam Y)
(%letg% ((l '()))
(%funcall% Lam)
(%not% (%member% Y l))
(%code% (set! l (cons (gp-cp YY S) l)))))

We just make sure to keep a list of older probe values and the function will success only when there is a new value that does not unify to any of the elements in the list. note again the use of letg.The last step is to combine all the other steps and do that with,

(%define% (%collect-2% Lam X Y L)
(%var% (YY)
(%call% ((Y YY))
(%uniq% (%/.% (%funcall% Lam X Y)) Y))
(%collect% (%/.% (%funcall% Lam X YY)) X L)))

Here Lam takes two values a probe value X and a by variable Y. %call% is a macro ovwe call above in step 1, where YY is the result and Y is the probe and the %uniq% etc is the Lam. %/.% is just making a closure e.g. a lambda with no arguments apart from standard ones. %var% makes a fresh new variable. The code should produce a list for each unique Y (the by variable) and when backtracking the next list with a new different Y will be made. This semantic is nice but can of cause be optimize for speed.

Have fun>

The logic at my fingertips

Guile log is an exploration of logic programming where one can choose to maintain the state in a stack or on a translation list like in kanren. Actually guile-log contains an implementation of the kanren interface that are as fast or faster as the original kanren distribution compiled on chicken ( a scheme environment).

The bad thing is that one need to track the state in more advanced application for which a state which is a combination of a mutual structure and the old cons list. Also this means some overhead and makes things complicated!

The good part is that one can combine the speed of the stack based solution found for example in gprolog with the generality and thread friendly solution that kanren provides.

If one like to just stick to the kanren approach one can take advantage to the stack in many ways which makes the coding experience richer. The basic principle is that you get a notion of going back and forth in time between different states and this walk can be traced to understand more about the program and enable some features

One of the features is to code with meta global variables (my naming). They behave as global variables with respect to computation e.g. storing values over backtracking cycles which means that one can enter sub backtracking regions that need to store for example a list of all results and use that list
as a value. The meta comes from the fact that these variables will behave as normal stack variables / kanren variables in the higher level backtracking where the list of results are used. Also when storing the state and restoring the state these variables will follow and be restored correctly.

Guile log is therefor ideal to use in an interactive proof solver for example because you can work from the guile prompt all the time and have all of guile log and guile scheme at your fingertips and e.g. be able store states try a branch go back, go forth and so on.

maybe one day I will port the coq prover to guile-log or something similar.

Aschm! not a cold just a combination of assembler and scheme

But most of my current time has been on hacking on the internals of guile. Wingo has started coding on a new VM for guile based on a register layout e.g. using local variable index in stead of pop and push to get and set local variable data. I on the other hand have ported SBCL's assembler (see aschm on gitorious) and implemented translations of the original VM and the new RTL VM to native code. Quite fun indeed and can show the speed benefits and overhead of using this.

maybe you can by a factor of 2x going to the RTL VM and another 3x compiling to native. To note here is that much of the overhead comes from the fact that all operations usually have some overhead, for example trying looping around and summing an incrementor e.g. calculating 1 + 2 + 3 ... would mean maybe 250Million additions per seconds for nativly compiled version. This is adding guile fixnums and there is some overhead of managing these datatypes, also the RTL feature is transfered to the native one as well e.g. the natively compiled code does not use the registers as much as one can do.

native logically yours


17 Aug 2012 (updated 17 Aug 2012 at 21:35 UTC) »
Its raining cats and cpu instructions

Right now I work with taking a specification of virtual operations, the guile rtl vm, and translate them to assembler code. The idea is to stack the resulting assembler instructions and compile to machine code in stead of using named gotos in C. Using this we will get less execution overhead but more code.

Some tests shows that simple loops like incrementing a counter and sum it as well as simple list operations, get a boost by 3-4 times by doing this. Of cause if expensive vm operations are used most of the time you will not gain much by such an approach.

The drawback of the method is that for example a simple addition may look like,

(define-vm-inst vm-add 79 ((U8_U8_U8_U8 dst x y))
(inst mov call-1 (local-ref x))
(inst mov call-2 (local-ref y))
(inst test call-1 2)
(inst jmp #:z slow:)
(inst test call-2 2)
(inst jmp #:z slow:)
(inst add call-2 call-1)
(inst jmp #:o slow:)
(inst sub call-2 2)
(inst mov (local-ref dst) call-2)
(inst jmp out:)
(inst mov call-2 (Q rsp))
(c-call scm_sum call-1 call-2)
(inst mov (local-ref dst) rax)

Disclaimer not debugged yet, but you get the picture 14 instructions!

The problem with verbose code like this is that it increases the compilation time of the code, and the size of the resulting programs. What I'd would like to have is the possibility to have macro instructions which get translated
to the specified code directly in hardware and just need to write

(inst mov call-1 (ref rbp 12))
(inst mov call-2 (ref rbp 13))
(inst macrocall 134)
(inst mov (ref rbp 14) rax)

e.g. just like a c call in amd64(Linux) but the macrocall will
be a hardware expansion on the cpu to the expansion found in slot 134. In all a good 3x decrease in instruction length and of much less complexity (no jumps).

A problem with this setup is that if you would want to use the machine registers effectively you may want to specify which registers you shall use in the macrocall like

(inst macrocall 134 call-1 call-2)

Another issue with this is that the same specific expansion code is located in the cpu for all processes and different processes might want to have their own expansion code in there. Not sure how to solve this, maybe each process have an id that describes which set of expansion it uses and when a new process start to execute on the core that key can be used to recognize if a different macro code should be loaded into the cpu when asked for. Of cause this can lead to expensive context switches. But I find it an interesting feature.

Have fun

8 Aug 2011 (updated 8 Aug 2011 at 20:49 UTC) »

A memory trick

I don't remember where I saw it but someone wrote that using CPS style prolog implementation is costly. And It is a little indeed. It will punish you with a factor of about 3x and the irregularities of garbage collection will hit you with a factor of 6x or more if it is done in passes. At least this is the timings I get on a prolog example written in C using a tail call trampoline and proper closures allocated from the heap. Now the same limiting feature would be present in a implementation in a Lisp or any other higher level language with proper closures and a working GC. The cool thing with the closures needed to do backtracking are that they don't need to allocate the closures on the heap. In stead one can maintain a stack to do the trick. I did implement a quick version using this method and the figures above came as a conclusion. So in principle there is a 4-5 fold of savings possible to go from the VM to Naitive compilation. A note here are that the overhead of the VM masks GC issues and there is not much gained from trying to squeeze extra vm instructions or compiler directives into generating special op-codes to allocate the closures from the stack in a special manner.

So how did I implement this. Well actually I have a small compiler written in scheme that output's c-code from a scheme like language, taking advantage of Alex Shinn's excellent fmt library. In this package there are element's to hide tail call trampoline semantics and closure generation to achieve close to scheme semantics. So in order to allocate the closere from a stack I needed an allocated array and companion pointer lam-stack, I added two extra macros, s-lambda, f-lambda. s-lambda allocates just the closure from the next bytes from the stack and are used in closures typically generated by (%and a b ...). f-lambda are the lambda that defines the next continuation at a junction. for this closure we can allocate it again from the stack and then when the lambda is run we simply set the stack pointer lam-stack to the next byte of the
end of the chunk of environment data for the closure that it needs to express the continuation. (that data is copied over, a strategy that works because we do not set! variables) In all, f-lambda semantics imply that we do not consume the stack and a quite small stack is enough for
many implementations.

The careful reader will recognize that this strategy will consume much more stack then needed. f-lambda sematics can be improved to do much less copying by noticing that the variables carried over to the next continuation follows a total order on the set of subsets of variables needed in the sequence of closures at a junction. This means that one can copy the variables only ones and then reuse the stackframe (so it is apparent that the current versions will need quite a lot of stack space at each junction. Also for the s-lambdas one can save some space by similar techniques. This can decrease the amount of copying and needed stack space.

How does the prolog macro package work? Here is %and%:

(define-syntax :and:
(syntax-rules ()
((_ w x) (Y w x))
((_ (cc cut fail s) (:cut:) . l)
(:and: (cc cut cut s) . l))
((_ (cc cut fail s) x . l)
(:let: ((ccc (:s-lambda: s (fail) (:and: (cc cut fail s) . l))))
(Y (ccc cut fail s) x)))))

and %or%:

(define-syntax :or:
(syntax-rules ()
((_ w x) (Y w x))
((_ (cc cut fail s) x . l)
(:let*: ((P (:scm-call: c_newframe))
(f (:f-lambda: s ()
(:scm-call: c_unwind P)
(or-work P (cc cut fail s) . l))))
(Y (cc cut f s) x)))))

Note here that cc is the continuation cut is a failure of an earlier junction. And here is how one defines a prolog function.

(:define: lam-stack (queens3 UnplacedQs SafeQs Qs)
;(:pp: "queens3 Unplaced ~a -> ~a" UnplacedQs SafeQs)
(:match: (UnplacedQs)
( _ (:var: (Q UnplacedQs1)
(:and: (:call: selectq Q UnplacedQs UnplacedQs1)
(:not: (:call: attack Q SafeQs))
(:call: queens3 UnplacedQs1
(:scm-call: c_cons Q SafeQs) Qs))))
(() (:=: SafeQs Qs))))

And we see that there are overhead and some carefulness in the coding but it's a lot of help over trying to write c-code that consists of 2000 lines of code compared to the 80 lines used to write the application. N.B. I didn't find a way to format the code snippets in a good way, sorry!

There are two more way's that can save time. Inlining and using fixnum arithmetics in stead of SCM based arithmetics. Also many new bottlenecks start to appear like the output printing routine. Anyway the conclusion are that a CPS + tail call trampoline is not to bad as a tool to do backtracking searches if done with the right compiler.

Have fun

25 Jul 2011 (updated 25 Jul 2011 at 21:44 UTC) »

Ordering the functional Ilands

note I use the word continuation below for a closures. This is not exactly what is meant by a continuation in scheme but the used closure function as a sort of delayed computation. Then
this computation object is passed as an argument to various functions until it under the right circumstances is evaluated. So call CPS semantics. Continuations can be implemented to a large degree with the help of this technique though.

Using the guile-2.0 virtual machine and adding a few opcodes targeting the evaluation of prolog programs yields quite good performance (from a few tests maybe less then 2x of gprologs VM). This is accomplished although continuations allocated from the heap are used. The conclusion is that this overhead is not too visible compared to the VM overhead. Actually the version I use combine setjmp like sematics for backtracking with continuation that represents a delayed computation and hence lead to slightly less need of heap compared to using continuations for the backtracking as well.

How are these features used? let's consider the quest of a tree search to solve for example the n-queens puzzle. What features do we have? Well for one thing we need variables that we can set to values that should be visible down the tree as we go out on a limb. If then a failure in the search is signaled we would like to go back and the movie for the values of a variable should be played backwards in order to restore a state at a branch point further up so the search can continue. Another thing is the need to sequence facts, first A should be true, then B and so on. lets write this as

(%and% A B ...)

Another basic building block is to model a branch point e.g. try A, if fail then try B etc. e.g.

(%or% A B ...)

finally we need a way to express that we fail if the fact is true e.g.

(%not% A)

In code %and% could be translated as

(compute (%and% A B) CC Fail)
(compute A (/.(Fail2) (compute B CC Fail2)) Fail)

where CC represents what should be computed after (%and% A B) have been computed (/.(arguments ...) code ...) can be looked at like a definition of a function with associated variables inside A B etc. captured e.g. the variable addresses is stored in them. This happens indirectly as in scheme or If you look at /. the creation as a creation of a object from a compute (used variables would then have to be passed explicitly on in the constructor arguments). Fail is the backtracker and is a continuation or can be a setjmp like backtracking. Let's assume that it is the continuation computation that represents whats need to be done if the computation fails. Now over to %or%

(compute (%or% A B) CC Fail)
(compute A CC (/.() (unwind) (compute B CC Fail)))

Note here that we need to unwind the variables int the failur computation ((/. ()) e.g. run the movie backwards before we restart computing the next branch. Also the failure computation does not have any arguments.
And finally %not%,

(compute (%not% A) CC Fail)
(compute A (/.(Fail2) (Fail)) (/.() (unwind) (CC Fail)))

Note here as well if we success e.g. A fails, we will undo any changes to the variables.

This is a closer look at some of the elements needed to construct prolog like semantics. There are something left like unifications and matching But this is a description to better understand the thoughts flowing out on my posts.

Now I have made a little macro package that outputs from a scheme like language pure c-code. I added closures without a correct set! semantics, allocating the closures from the heap. Also a tail call trampoline framework was added. The generated code is three times slower then compiled gprolog and also it's quite obvious that a lot time is lost due to a hefty use of gc and the heap. So this system did not have a significant improvement compared to the VM approach with a setjmp semantics replacing the failure continuation computation. My next quest will therefore be to modify the macro package to allow allocating the closures from a stack instead. Here the key fact to note are that the Failure continuation should be able to reset to the same position on the stack at a branch. In all there should be hope that memory consumption can be bounded enough to make it practical.

Have fun

A parallel brain is more expressive then an unparallel one

Assume that we have deduced a state S and there are two possible choices S1 and S2 to go from there how to zip them together?

One idea is to assume S1 outputs z1 and S2 outputs z2 and just combine these outputs e.g.

(zip-1 (z1 z2)
     (subshell S1 z1)
     (subshell S2 z2))
Here we create z1 and z2 on an outer stack, then run S1, it produces a value into z1 that are deduced on an inner stack. Then it will keep z1 and revert back to the initial state but store a continuation that are reinstated at back-tracking. This will be able to mimic generator behavior through back tracking. It is safe code because the interface is clear and has a potential performance bottleneck in that we need to reinstall the state of S1 and S2 at back tracking.

The other idea is to consider the stack as a DAG and use two branches of the stack describing the deduction S1 and then deduction S2. First S1 is done, then S2. and the state will be done on-top of each other but the action will be stored in branches e.g. a storage of a variable pointer and a value. But at backtracking, then we will just first backtrack S2 and by doing this not touch the other stack branch. Then we will backtrack S1 and not do the undos on the S2 stack branch. If there are a variable touched in both S1 and S2, with backtracking touching it only in S1 or only in S1 unpredictable things can happen. So this method is faster but can produce strange bugs.

Again parallel generation of sequences can be simulated using this trick.

In all, zip patterns call's for a more delicate stack engine then the common stack implementation one need to manage a forest of DAG's that should support continuation style of separation and storage. linked structures are attractive here due to it's simplicity in these situations, on the other hand the speed of allocations from a stack represented by an array is also attractive. Maybe a combination.

Life on a t-shirt

Oh and now over to that jolly song from all of yo together on that crucifix hill in easter time, one two three ...

One of my favorite t-shirt ideas is to have all the religious celebrities together on crucifixes, (mohammed pained as a cloud with perheps lips showing that he sings along) and all of them singing and smiling :-) And the song should of cause be the one from the life of brian - Always look on the bright side ...)

This picture tell the simple minded theo/philo-egoists in the world to fuck off to some deep cave and keep us hard working people to continue with what we are good at, or, Hey teacher, We don't need your silly religious war, leave them kids alone, or simply C.U. cooperate stupid.

Over to some cool mathematical play. Did you know that the fundamental law's of force physics is non-linear? This have a kind of cool direction, let's call it the twilight direction. The thing is that a common theme to express solution to non-linearity, especially when the nonlinear part is weak, is to first just solve for the linear solution X, then solve for the twighlight solution T, which we get by solving the linear equations again but with a source that represent the nonlinear part applied to X and construct the solution as X + T. Similarly this technique can under a fix-point theorem result in an infinite expansion that lead to a solution to the nonlinear equation.

So, basically T is small ripples that lives on top of the real fat and dirty reality called X. And if the connection is really weak we would never detect T. We should have seen it already if that was the case right! Now over to crystallography. What's cool with this technology is that if you can build a crystal of say hemoglobin, (e.g. the same molecule is reproduced over and over again in a periodic lattice), then you can sort of overlay picture signals of all entities in the crystal and by statistical laws the details get many times more clear then by just looking at one single entity with some kind of microscope. So consider some kind of periodic twighlight ripple T. and consider sensors in billions of places throught a volume, wouldn't that magnify the probability of detection considerable, I wonder if humans have build any similar device to try detect the twilight zone, I know the homo crackpottus have tried it in another universe somewhere.

Oh, I put a little "easter egg" somewhere in _this_ universe so that you can perhaps understand my math (yes a little prose attached to the math) ;-) ... happy reading


Alex Shinn, has written a library for scheme that does formatting tasks in a functional style, the fmt library. In this library there is a package for outputting c code. Let's play with that.

A next step might be to be able schemify c code. e.g. introduce <define>, <let*>, <begin>, <if> and <recur> + the operators. The innovation needed to do this is to make sure that everything return values and are able to be composed together with some kind of "sewing" mechanism.

The idea of attacking this is to try keep everything functional as far as possible and then at the final step add syntactic sugars in the form of macros. When trying to do this one realizes that we need two phases of evaluation. The reason are this, consider a function application:

int f(int),
f((<let*> ((a (+ b c))) (<if> q a b)) + b)?
Now the idea of modelling is to let the statments evaluate to a lambda that takes as an argument #f or a symbol. The lambda applied to a symbol will mean that the result of that expression will generate c-code where the tail expressions will set the symbol to the return result. so we do stuff like,

(define (f-if p x y)
    (lambda (ret)
       (let ((pred (gensym "pred"))
           (c-var 'int pred)   ; define pred integer
           (p pred)            ; execute p expression
                               ; pred is the result
           (c-if pred (x ret) (y ret))))))
This will illustrate the technique (we are using autoquoting features in the macro package). Over to the function application. Here we conclude that in order to be general we need to use

   (int arg)
   ((((<let*> ((a (+ b c))) (<if> q a b)) + b)
   (c= ret `(f ,arg))
E.g. we introduce an arg symbol to be able to set it. Doing this we need to know about the function signature. Designing the <define> as a function means that we are not able to register the function signature before the body is evaluated and the function signature needed in case of recursion. So one need one extra lambda dereference. I thught that was too complicated though and used macros in stead to accomplish correct registration order.

The function example lack some wow factor. But for the loop construct in clambda, the <recur> statement, it's a must. Here is how the idea of this works

(<recur> loop ((int n 100) (int s 0)
    (<if> (< n 0)
          (<next> loop (+ s 314))))
Aproximate translation
 int n = 100;
 int s = 0
 if (n < 0)
    ret = s;
    s = s + 314;
    goto loop;

This is kind of handy loop construct and again one need to register the loop symbols this time in order to be able to expand the <next> statement later on.

This is about 300 lines of code. For 600 I guess you would be able to introduce a good enough c to produce workable code with syntax line numbers introduced as well to be able to debug after gcc has found your bugs.

Now this is not that complicated and everything is just a quite simple idea of syntactic sugars. But this will enable scheme macro facilities to your c-code (for one thing you will be able to track source code line numbers to be able to debug your macros correctly. Also going between clambda and pure scheme is a smaller step then between c and scheme this means that you can keep a large part of say guile c-code both in c and scheme at the same time.

Have fun

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