Pure Functional Memoization

There are many computation problems that resonate through the ages. Important Problems. Problems that merit a capital P.

The Halting Problem…

P versus NP…

The Fibonacci sequence…

The Fibonacci sequence is a super-important computation Problem that has vexed mathematicians for generations. It’s so simple!

fibs 0 = 0 fibs 1 = 1 fibs n = (fibs $ n - 1) + (fibs $ n - 2)

Done! Right? Let’s fire up ghci and find out.

*Fibs> fibs 10 55

… looking good…

*Fibs> fibs 40

… still waiting…


… Well that sure took an unfortunate amount of time. Let’s try 1000!

*Fibs> fibs 1000 *** Exception: <<You died before this returned>>

To this day, science wonders what fibs(1000) is. Well today we solve this!


The traditional way to solve this is to use Memoization. In an imperative language, we’d create an array of size n, and prepopulate arr[0] = 0 and arr[1] = 1. Next we’d loop over 2 to n, and for each we’d set arr[i] = arr[i-1] + arr[i-2].

Unfortunately for us, this is Haskell. What to do… Suppose we had a map of the solutions for 0 to i, we could calculate the solution for i + 1 pretty easily right?

fibsImpl _ 0 = 0 fibsImpl _ 1 = 1 fibsImpl m i = (mo + mt) where mo = Map.findWithDefault undefined (i - 1) m mt = Map.findWithDefault undefined (i - 2) m

We return 0 and 1 for i = 0 and i = 1 as usual. Next we lookup n – 1 and n – 2 from the map and return their sum. This is all pretty standard. But where does the map come from?

It turns out that this is one of those times that laziness is our friend. Consider this code:

fibs' n = let m = fmap (fibsImpl m) (Map.fromList (zip [0..n] [0..n])) in Map.findWithDefault undefined n m

When I first saw this pattern (which I call the Wizard Pattern, because it was clearly invented by a wizard), I was completely baffled. We pass the thing we’re creating into the function that’s creating it? Unthinkable!

It turns out that this is just what we need. Because of laziness, the fmap returns immediately, and m points to an unevaluated thunk. So, for i = 0, and i = 1, fibsImpl will return 0 and 1 respectively, and the map will map 0 -> 0 and 1 -> 1. Next for i = 2, Haskell will attempt to lookup from the map. When it does this, it will be forced to evaluate the result of i = 0 and i = 1, and it will add 2 -> 1 to the map. This will continue all the way through i = n. Finally, this function looks up and returns the value of fibs n in linearish time. (As we all know, Map lookup isn’t constant time, but this is a lot better than the exponential time we had before)

So let’s try it out.

*Fibs> fibs' 1 1 *Fibs> fibs' 10 55 *Fibs> fibs' 40 102334155

… so far so good…

*Fibs> fibs' 100 354224848179261915075 *Fibs> fibs' 1000 43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875 *Fibs> fibs' 10000 33644764876431783266621612005107543310302148460680063906564769974680081442166662368155595513633734025582065332680836159373734790483865268263040892463056431887354544369559827491606602099884183933864652731300088830269235673613135117579297437854413752130520504347701602264758318906527890855154366159582987279682987510631200575428783453215515103870818298969791613127856265033195487140214287532698187962046936097879900350962302291026368131493195275630227837628441540360584402572114334961180023091208287046088923962328835461505776583271252546093591128203925285393434620904245248929403901706233888991085841065183173360437470737908552631764325733993712871937587746897479926305837065742830161637408969178426378624212835258112820516370298089332099905707920064367426202389783111470054074998459250360633560933883831923386783056136435351892133279732908133732642652633989763922723407882928177953580570993691049175470808931841056146322338217465637321248226383092103297701648054726243842374862411453093812206564914032751086643394517512161526545361333111314042436854805106765843493523836959653428071768775328348234345557366719731392746273629108210679280784718035329131176778924659089938635459327894523777674406192240337638674004021330343297496902028328145933418826817683893072003634795623117103101291953169794607632737589253530772552375943788434504067715555779056450443016640119462580972216729758615026968443146952034614932291105970676243268515992834709891284706740862008587135016260312071903172086094081298321581077282076353186624611278245537208532365305775956430072517744315051539600905168603220349163222640885248852433158051534849622434848299380905070483482449327453732624567755879089187190803662058009594743150052402532709746995318770724376825907419939632265984147498193609285223945039707165443156421328157688908058783183404917434556270520223564846495196112460268313970975069382648706613264507665074611512677522748621598642530711298441182622661057163515069260029861704945425047491378115154139941550671256271197133252763631939606902895650288268608362241082050562430701794976171121233066073310059947366875 *Fibs> fibs' 100000 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Neat. Even that last one only took a few seconds to return!

Aeson Revisited

As many of you know, the documentation situation in Haskell leaves something to be desired. Sure, if you are enlightened, and can read the types, you’re supposedly good. Personally, I prefer a little more documentation than “clearly this type is a monoid in the category of endofunctors”, but them’s the breaks.

Long ago, I wrote about some tricks I found out about using Aeson, and I found myself using Aeson again today, and I’d like to revisit one of my suggestions.

Types With Multiple Constructors

Last time we were here, I wrote about parsing JSON objects into Haskell types with multiple constructors. I proposed a solution that works fine for plain enumerations, but not types with fields.

Today I had parse some JSON into the following type:

data Term b a = App b [Term b a] | Var VarId | UVar a

I thought “I’ve done something like this before!” and pulled up my notes. They weren’t terribly helpful. So I delved into the haddocs for Aeson, and noticed that Aeson's Result type is an instance of MonadPlus. Could I use mplus to try all three different constructors, and take whichever one works?

instance (FromJSON b, FromJSON a) => FromJSON (Term b a) where parseJSON (Object v) = parseVar `mplus` parseUVar `mplus` parseApp where parseApp = do ident <- v .: "id" terms <- v .: "terms" return $ App ident terms parseVar = Var <$> v .: "var" parseUVar = UVar <$> v .: "uvar" parseJSON _ = mzero

It turns out that I can.

Making C++ Exceptions Less Terrible

If you’re a fan of doingmyprogramming, you’ve likely heard me opine on the subject of exceptions. If you’re new around here, I’ll spare you a link to some required reading and just let you know: they’re poop. With the exception of Java and it’s checked exceptions, they just represent secret ways that a function can crash your whole program. Just about nobody gets it right, not C++, not C#, and the worst offender is probably my beloved Haskell where you can’t even catch them unless you’re in the IO monad.

C++ used to have an annotation (throw()), where you could specify what exceptions a function throws, and if none are specified then the function cannot throw. This was considered poor form and deprecated long before I came on the scene. It turns out that there is good news though; throw() wasn’t forgotten! It was replaced by noexcept, and that is what we’ll be talking about today.

What is noexcept?

noexcept is an annotation that basically says “this function will never throw an exception”. You can think of it as being like const. If you some class method:

void myClass::isPure() const;

… then you know that, whatever it does, the state of the object will not be mutated. Similarly, if you see some function:

void neverThrows() noexcept;

… then you know that, whatever it does, the function will never throw an exception. Optimization implications aside, this is great because if you see a function declared noexcept, then you know for sure that you don’t have to put it in a try/catch block. If we ever start living in a world where people use this annotation, then a function not marked noexcept is a function that likely throws, and we can act accordingly without having to read through mountains of likely incorrect or incomplete documentation!

Specifically, the meaning of noexcept (according to Microsoft) is:

noexcept ( and its synonym noexcept(true)) specify that the function will never throw an exception or allow an exception to be propagated from any other function that it invokes either directly or indirectly. More specifically, noexcept means the function is noexcept only if all the functions that it calls are also noexcept or const, and there are no potentially evaluated dynamic casts that require a run-time check, typeid expressions applied to a glvalue expression whose type is a polymorphic class type, or throw expressions.

The Burning Quetion

So I read that blurb, and thought to myself: “…only if all the functions it calls are noexcept? So, I can’t catch all possible exceptions and be noexcept?”. I felt despair wash over me as C++ got something wrong yet again. Then I thought “there’s no way that’s correct… To the Googler!”

Of course, the Googler didn’t help my case. I decided to give it a shot. First, let’s consider two functions: one that throws and one that is noexcept:

static void doesThrow() noexcept(false) { std::cerr << "Entering doesThrow..." << std::endl; throw 42; std::cerr << "Still in doesThrow. Up is down, cats and dogs living together!" << std::endl; } static void catchesAll() noexcept(true) { std::cerr << "About to enter the try/catch..." << std::endl; try { doesThrow(); } catch (...) { std::cerr << "Caught the exception..." << std::endl; } std::cerr << "Exited the try/catch..." << std::endl; }

doesThrow is annotated noexcept(false), which is the same as saying “This function can throw an exception.” And sure enough, it throws in 100% of its code paths.

catchesAll is annotated noexcept(true), which is just the long form of noexcept. noexcept accepts arguments that evalutate to bool, and compile-time magic happens to allow conditional noexcept-ness. noexcept(expr) can also be used in an if/then/else to conditionally do something based on if some expr is noexcept.

In catchesAll, we have a try/catch block that prevents the exception thrown by doesThrow() from propagating out of catchesAll(). Does this work? Let’s write a test harness and see:

int main(int argc, char ** argv) { std::cerr << "About to call catchesAll..." << std::endl; catchesAll(); std::cerr << "Managed to not die!" << std::endl; }

… if we compile it with gcc (with -std=c++11) and run it, we see that all is well!

About to call catchesAll... About to enter the try/catch... Entering doesThrow... Caught the exception... Exited the try/catch... Managed to not die!

You may be wondering what happens if we don’t catch the exception. Let’s find out! If we remove the try/catch, but leave everything else the same:

About to call catchesAll... About to enter the try/catch... Entering doesThrow... terminate called after throwing an instance of 'int' Aborted (core dumped)

We see that the exception propagated out of catchesAll() and caused a core dump. This is the expected behavior. In fact, there’s no way we could have prevented this. If some function marked noexcept throws, then that function has a bug. If you wrote it, you have to fix it. If somebody else wrote it, then they have to fix it. There’s no band-aid that will keep it from crashing your program. If you find this happening often, then you should consider finding an alternative library that does what the broken dependency claims to do, since they clearly don’t know what they’re doing…

Operator Overloading in C++

My thoughts on Operator Overloading are hardly a secret. Some languages, such as Haskell, support it in a sane manner. Some languages, such as C++ don’t. Either way though, I’m not a fan.

That said, there is one case where I support it; when what you are trying to overload an operator to do makes sense for your type. In other words, if it makes sense to multiply an Employee object, feel free to overload the multiplication operator. Since we don’t usually model mating when we implement an employee object, I don’t usually approve.

Multiplying a Fraction

However, if we have a fraction class, that has a numerator and denominator component, overloading the multiplication operator suddenly becomes reasonable:

frac<T> operator *(const frac<T> & rhs) const { return frac<T>(numer * rhs.numer, denom * rhs.denom); };

In C++, many of the built in operators are available to be overloaded. You can do basically anything in an operator overload, but if you do, that makes you a bad person. Here we have a straightforward implementation, that I’m going to assume you don’t need to have explained to you. But let’s talk const correctness.

In this signature, the const appears twice. The right hand side argument is a const reference. Since this is a reference, no copy occurs, but with references can come mutability. The const here prevents any mutation from occurring. There is also a const at the end of the signature. That means that this function cannot modify the this pointer.

This function returns a new frac<t>, so no need for const there.

Convert a Fraction to a Double

Next, you can overload the casting operator, and if you are a good person you’ll use this for type conversions. Let’s implement (double) for our fraction:

operator double() const { return (double) numer / denom; }

This code is pretty straight forward. We divide the numerator by the denominator, and cast it to double in case integer division occurs. Cast operator overloads don’t have a return type, which is a bit strange but something we can work with.

Streaming operators

Finally, I’d like to talk about streaming operator overloads. For any overloaded operator, there are two forms: member and non-member. Member overloads take n - 1 arguments, where the minus 1 is the current object (and the left hand side). Non member overloads can’t access the this pointer, so they need to take the left hand side as a parameter.

Member overloads obviously have access to private class members, which makes them more powerful than non-member. Which leads us to the streaming operators. Streaming operators need to return std::ostream &, so they must be non-member. The problem is that we want to print the private numerator and denominator fields. The solution? make it a friend:

friend std::ostream & operator <<(std::ostream & lhs, const frac<T> & rhs) { lhs << rhs.numer << '/' << rhs.denom; return lhs; };

With that problem solved, this function becomes pretty straight forward. We stream the fields, formatted correctly, into the left hand side std::ostream &, then we return the left hand side argument for further use by the next part of the streaming chain.

Back in the Saddle with C++

Next quarter, I’ll be taking a graphics programming class. It was a year ago to the day that I last wrote about getting a makefile set up for OpenGL development. As you can imagine nothing came of it. Why? The obvious answer is that OpenGL is hard.

Secondary answers include the fact that almost all of the literature is assuming C++, and I was swimming against the tide trying to use C. Well, I won’t have a choice this time, the class is in C++, so I’m stuck with my old nemesis again.

What follows in the next few posts is an attempt to refamiliarize myself with C++. It’s been a while since I’ve done any significant work in C++ and a lot has happened. I’ll be going over a lot of basic stuff, so that I have a centralized crash course for myself when I’m stuck trying to remember if it’s const &, & const, & const &, or co&n&st (stranger things have happened).


First up: namespaces. Namespaces are a formalization of the c convention of writing functions called stuff like dmp_doStuff. First we can declare our namespace:

namespace dmp { void doStuff() }

…and we can call this function by doing dmp::doStuff(). This may involve an extra keystroke, but we can also use it like so:

using namespace dmp; doStuff(); doStuff(); doStuff(); while (stuffUndone()) doStuff;

<Template> Classes

Now that we got that out of the way, let’s get into the meat of it: template classes. First up, due to some incredibly unfortunate reason, a C++ template class has to go in the header file. This sad fact is one of C++’s many transgressions against society, but we’ll not worry about that for now.

To declare a class (inside of a namespace if you’re a good citizen):

namespace dmp { template <typename T> class frac { public: /* public stuff goes here */ private: T numer; T denom; /* and your privates go here */ }; }

template <typename T> would be omitted if this weren’t a template class. Here we declare a class frac with one generic type T. Our class has two private fields of type T. As we all know, class members are private by default, but I don’t like to rely on “default behaviors”, so it doesn’t hurt to make it explicit. Let’s add some public constructors:

frac(T inNumer, T inDenom) : numer(inNumer), denom(inDenom) {}; frac(const frac<T> & toCopy) : numer(toCopy.numer), denom(toCopy.denom) {};

Here we use initializer lists to populate the numerator and denominator. The first constructor takes two arguments of the same type (T), and the second is the copy constructor.

According to Microsoft, initializer lists are more efficient than assignment in the function body, which is why we prefer them here. There are certain cases where you must use an initializer list, and cases where you cannot use them. Your compiler will tell you if you mess this up.

Finally, let’s add a destructor:

~frac() {};

Here we have a marvel of software engineering. Since we really don’t have anything to destroy, we can let it do nothing. It should be noted that this destructor is not virtual, so it’s not really safe to extend this class (more on this another day).


Honestly, namespaces and constructors/destructors are two of my favorite features of C++. Simple, but helpful in eliminating boilerplate. I almost might be convinced to compile “C” with a C++ compiler to get these features. Coming up will be some features that I’m a bit more iffy about, such as operator overloading. however, when used appropriately, these things can also be good, more to come!

Pretty Printing our Pretty Program with Pretty

Tell me if you’ve heard this one before. You’re making some complicated data structure in Haskell. Something like the following:

data Expr = EVal Int | EIdent String | EFunc String [String] [Expr] | ECall String [Expr] | ELet String Expr | EWhile Expr [Expr] | EShizzy Expr Expr Expr | EIfElse Expr Expr Expr | EReturn Expr deriving (Show)

You throw that deriving (Show) because it’ll totally help you debug! Time goes by, and you use your fancy Expr to write a robust imperative function:

test = EFunc "testFunc" ["gatito", "moogle"] [ELet "foo" $ ECall "petCat" $ [EIdent "gatito", EIdent "moogle"], EWhile (ECall "catsPlacated" []) [EShizzy (EIfElse (EVal 5) (ELet "foo" $ ECall "malloc" [ECall "sizeof" [EVal 2]]) (ECall "petCat" $ [EIdent "gatito", EIdent "moogle"])) (ECall "petCat" $ [EIdent "gatito", EIdent "moogle"]) (ELet "foo" $ ECall "safeLeakMemory" []) ], EReturn $ EIdent "foo" ]

You’re still good, you got Rainbow Delimiters to keep that all sorted. Now, more time goes by and you’re in the repl and something doesn’t work. What was this test thing again?

*Shizzy> test EFunc "testFunc" ["gatito","moogle"] [ELet "foo" (ECall "petCat" [EIdent "gatito",EIdent "moogle"]),EWhile (ECall "catsPlacated" []) [EShizzy (EIfElse (EVal 5) (ELet "foo" (ECall "malloc" [ECall "sizeof" [EVal 2]])) (ECall "petCat" [EIdent "gatito",EIdent "moogle"])) (ECall "petCat" [EIdent "gatito",EIdent "moogle"]) (ELet "foo" (ECall "safeLeakMemory" []))],EReturn (EIdent "foo")]

Well that’s unfortunate. What do we do now? Aside from the fact that this is completely useless to us, this certainly wouldn’t be OK to print out to a user…

The good news is that pretty printing is a snap thanks to the HughesPJ Pretty Printer. One simply has to implement the Pretty class for one’s type, and then they can get some reasonable output:

testFunc(gatito moogle) { let foo = petCat(gatito moogle) while(catsPlacated()) { +-- { if (5) let foo = malloc(sizeof(2)) else petCat(gatito moogle) } +++ + + +++ { petCat(gatito moogle) } +++ + + +++ { let foo = safeLeakMemory() } +-- } produceResult foo }

Let’s see how we get there, constructor by constructor.

EVal and EIdent

First up are the easy ones: EVal and EIdent, which represent bare values:

instance Pretty Expr where pPrint (EVal v) = int v pPrint (EIdent s) = text s

First we have to declare our instance, but next we can just print the values. int pretty prints an integer, and text pretty prints a string.

EFunc and ECall

Next we have function calls and definitions:

pPrint (EFunc n a b) = text n <> (parens $ hsep $ fmap text a) <+> lbrace $+$ (nest 2 $ vcat $ fmap pPrint b) $+$ rbrace pPrint (ECall n a) = text n <> (parens $ hsep $ fmap pPrint a)

There are some operators here that require explanation. All of these operators have type (Doc -> Doc -> Doc ), and are used to combine two Doc into one. A Doc is the type of thing that can be pretty printed, which is returned by calls to pPrint and various other primitive pretty printing functions provided by the library (like text and int)

<> glues the thing on the left next to the thing on the right with no space in between, where <+> does the same, but leaves 1 space in between. Similarly, $$ and $+$ glues the thing on the right to the bottom of the thing on the left. $+$ allows the right hand side to overlap with the left (more on this later). There are also hsep, and vcat which takes lists of Doc. These functions work similarly to <>, $$ and friends; they glue Docs together horizontally and vertically. *cat are the non-plus variants, and *sep are the plus variants.

Other functions introduced here are parens, which takes a Doc and surrounds it by parens. lbrace and rbrace insert { and } respectively. There is a braces function as well, but I didn’t use it because I want to control how the braces are shown. Finally, we have nest, which indents its argument. This function is great because it takes into account outer nest calls, so we can get arbitrary nesting.

ELet, EWhile, EIfElse, and EReturn

pPrint (ELet n e) = text "let" <+> text n <+> equals <+> pPrint e pPrint (EWhile p b) = text "while" <> (parens $ pPrint p) <+> lbrace $+$ (nest 2 $ vcat $ fmap pPrint b) $+$ rbrace pPrint (EIfElse p t f) = text "if" <+> (parens $ pPrint p) <+> pPrint t $+$ text "else" <+> pPrint f pPrint (EReturn v) = text "produceResult" <+> pPrint v

Nothing particularly fancy here. in ELet, we have equals which inserts an equals sign and we have more nesting throughout. As you can see, this is very simple once you get the hang of it; there are few abstract instances involved, and most functions do what they say.


As we all know, any programming language that doesn’t have Shizzy statements isn’t worth knowing, so of course I implement them:

pPrint (EShizzy u d t) = onFire $+$ (nest 6 (lbrace $+$ (nest 2 (pPrint u)) $+$ rbrace)) $+$ noRules $+$ (lbrace $+$ (nest 2 (pPrint d)) $+$ rbrace) $+$ noRules $+$ (nest 6 (lbrace $+$ (nest 2 (pPrint t)) $+$ rbrace)) $+$ onFire where onFire = nest 2 $ text "+--" noRules = nest 2 $ (text "+++" $$ text "+ +" $$ text "+++")

This produces the following output as we’d expect:

+-- { if (5) let foo = malloc(sizeof(2)) else petCat(gatito moogle) } +++ + + +++ { petCat(gatito moogle) } +++ + + +++ { let foo = safeLeakMemory() } +--

and this gives me a good opportunity to demonstrate the difference between $$ and $+$. Let’s swap all the plus variants out:

onFire $$ (nest 6 (lbrace $$ (nest 2 (pPrint u)) $$ rbrace)) $$ noRules $$ (lbrace $$ (nest 2 (pPrint d)) $$ rbrace) $$ noRules $$ (nest 6 (lbrace $$ (nest 2 (pPrint t)) $$ rbrace)) $$ onFire where onFire = nest 2 $ text "+--" noRules = nest 2 $ (text "+++" $$ text "+ +" $$ text "+++")

Now, the pretty printer outputs the following:

+-- { if (5) let foo = malloc(sizeof(2)) else petCat(gatito moogle) } +++ + + +++ { petCat(gatito moogle) } +++ + + +++ { let foo = safeLeakMemory() } +--

Basically, if something is nested far enough that it would appear after the end of the left hand side argument, it is pasted to the right of it instead of below it. Honestly, it’s pretty strange behavior to me, I say just avoid $$ for the most part and you’ll be fine. It’s not a huge stretch for me to think of a use for this, but I think I’d just use <> or <+> if I wanted this to happen.

Regardless, I’ve worked on projects that use this pretty printing library before, and now that I’ve given it a shot I know why!

Into The (Local) Cloud!

I, and I’m sure many of you use Github. If not, and you care to get started, I’ve written about this before. However, sometimes you don’t want to work in the open. If this is the case, then you likely aren’t going to be using Github. Sure, you can pay for private repos, or you can use alternatives like Bitbucket that provide free private repos. Personally, I prefer to keep my private things out of The Cloud (TM).

I have a headless computer hooked up to my home network running Debian that I use as a file server. I find it convenient to have my various projects that aren’t on Github in git repositories on my server. It provides a backup, and makes it easy to keep my desktop and laptop in sync. So, let’s talk about that.

Making The Repository

This is a pretty straightforward process. Much like the usual ways of making a local repository, we’ll be using git-init, however, we need to create a bare repository, otherwise we’ll have issues pushing and pulling. To do this, enter the following in the directory on the server where you want the repository to live:

git init --bare [repo_name]

Where [repo_name] is the name of the repository. A folder with this name will be created in the current directory.

This repository is special; you cannot work directly in it, you must clone and use push/pull.

Meanwhile, on Our Workstation

Back on the machine where we’ll be working, we first need to clone our newly created repository:

git clone [username]@[server]:[path_to_repo]

Here, [username] is your user on the server that has SSH access, [server] is the IP address or hostname of the server, and [path_to_repo] is the location of the repo on the server. When you do this, you’ll get a warning about cloning an empty repo which you can ignore. After all, of course it’s empty, you just made it!

…and guess what? That’s all there is to it! You can push and pull as normal at this point, and should be ready to go.

But DMP Guy, I Already Have A Repo!

So, you already got a repo, and you want to move it to a central server? This isn’t much more difficult. First, get the repo itself to the server somehow. I recommend SFTP:

sftp [username]@[server] sftp> put [compressed_repo] sftp> bye

Where [username] is an account and [server] is the server and [compressed_repo] is your repo directory compressed in your favorite manner.

After this is done, ssh to the server, then find and uncompress the repo. Next we make a bare repo out of it:

git clone --bare [orig_repo] [bare_version]

Here, [orig_repo] is the original repo that you just extracted, and [bare_version] is the name you want to give the new bare version. After this is done, you can rm -rf [orig_repo], and then clone and use [bare_version] as described above.

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