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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!

Getting Started With GLib in Emacs

Recently, I decided to start doing some C. In the past, I’ve used GLib in my C programs, and I’m a fan. I decided that I’d like to use GLib in my current endeavors. All that said, before I can use it, I have to be able to build it. Unfortunately, nothing in my life just works, so it took some configuring.

The Makefile

In my last post, I talked about creating a Makefile, and walked through it. I forgot one huge thing though: pkg-config!

Previously in DMP Photobooth, I used pkg-config to manage my library compiler flags. To that end, let’s make some changes to the Makefile I wrote previously. First, let’s refer back to what I wrote before:

COMPILE_FLAGS = -c -g -Wall -Wextra -std=c11 $(OPTIMIZE_LEVEL) LINK_FLAGS = -g -Wall -Wextra -std=c11 $(OPTIMIZE_LEVEL) LINKER_LIBS = -lSDL2 -ldl -lGL

It’s pretty straightforward. I have a compile flag set for compiling a .o, and for compiling a program. I also have a LINKER_LIBS variable to pass to the compile command. This isn’t part of the COMPLIE/LINK_FLAGS because the sources and object code being compiled must appear first or GCC complains. Now, let’s take a look at the new snippet:

COMPILE_FLAGS = -c -g -Wall -Wextra -std=c11 $(OPTIMIZE_LEVEL) \ $(shell pkg-config --cflags $(PKG_CONFIG_LIBS)) LINK_FLAGS = -g -Wall -Wextra -std=c11 $(OPTIMIZE_LEVEL) \ $(shell pkg-config --cflags $(PKG_CONFIG_LIBS)) PKG_CONFIG_LIBS = glib-2.0 gl sdl2 MANUAL_LIBS = -ldl LINKER_LIBS = $(MANUAL_LIBS) $(shell pkg-config --libs $(PKG_CONFIG_LIBS))

Things are getting just a bit more complicated now. You’ll notice there are three LIBS related variables. PKG_CONFIG_LIBS is the list of libraries to be passed to the pkg-config command. MANUAL_LIBS, as the name implies, is a list of manually configured -l strings. For the life of me, I couldn’t figure out what to pass to pkg-config to get it to spit out -ldl, so I’m forced to do it this way.

Regardless, LINKER_LIBS now contains the MANUAL_LIBS, and the output of $(shell pkg-config --libs $(PKG_CONFIG_LIBS)) which produces the necessary -l strings for all the PKG_CONFIG_LIBS.

On top of that, I’ve added the output of $(shell pkg-config --cflags $(PKG_CONFIG_LIBS)) to the COMPILE_FLAGS and LINK_FLAGS. This will ensure that if any pkg-config library needs special compiler flags, that they get used.

Great, now that’s done. A quick make, and everything seems to be working. We’re in business! …right?

Convincing Flycheck

If only it could be that easy. I created a new source and entered the following:

#include <glib.h>

Flycheck wasn’t convinced though; it put some red jaggies under this, and a quick mouse over of the error shows that flycheck doesn’t think that file exists. I began getting deja vu. After some googling, I determined that I can add arbitrary paths to flycheck-clang-include-path (I’m using the flycheck clang checker, if you’re using gcc this variable is going to be different. I’m guessing flycheck-gcc-include-path) to rectify the issue. To do this, enter:

M-x customize-variable [ENTER] flycheck-clang-include-path [ENTER]

This will get you a customize window for this variable. I added the following:

/usr/include/glib-2.0 /usr/lib/x86_64-linux-gnu/glib-2.0/include

…and things seem to be working fine. That said, I imagine if I get more involved in the GLib stack, I’m going to have to add all of these guys:


Not a huge deal, but I’ll cross that bridge when I come to it.

Checking Our Heads With Liquid Haskell

Consider this function:

headInTail :: [a] -> [a] headInTail l = (tail l) ++ [head l]

Pretty straightforward, right? It takes a list, extracts the head and sticks it in the tail. Surely you’ve written something like this before. It should be fine, right?

*Main> headInTail [1,2,3] [2,3,1]

…checks out. Let’s try a few more:

*Main> headInTail "hello" "elloh" *Main> headInTail ["cat"] ["cat"]

…good. And the moment we’ve all been waiting for:

*Main> headInTail [] *** Exception: Prelude.tail: empty list

Oh yeah, head and tail don’t work for empty lists… Normally, we have some choices on how to proceed here. We could wrap the function in a Maybe:

maybeHeadInTail :: [a] -> Maybe [a] maybeHeadInTail [] = Nothing maybeHeadInTail l = Just $ headInTail l

…which introduces an annoying Maybe to deal with just to stick our heads in our tails. Or, we could just do something with the empty list:

headInTail :: [a] -> [a] headInTail [] = [] headInTail l = (tail l) ++ [head l]

…but what if returning the empty list isn’t the correct thing to do?

Another choice is to document this behavior (as head and tail do), and just never call headInTail []. But how can we guarantee that we never attempt to call this function on an empty list? Shouldn’t this be the type system’s problem?

Unfortunately, not all is roses and puppies. Sometimes the type system cannot help us. Sometimes somebody thought it’d be a good idea to use Haskell’s Evil exception system. Whatever the case, there are tools to help us.

Liquid Haskell

Liquid Haskell is a static code analysis tool that is used to catch just these sorts of problems. Liquid Haskell allows us to define invariants which will be enforced by the tool. Liquid Haskell is a research project that is still in development. As such, it has some rough spots, however it’s still very much capable of helping us with our problem here. But before we begin, we need to get the tool installed.

To install the tool, execute:

cabal install liquidhaskell

…simple right? Unfortunately, we’re not quite done. We need to install an SMT solver. This tool is used by Liquid Haskell. Currently, the tool defaults to Z3. I’m not sure how to use a different solver (and Z3 works just fine), so I suggest you you Z3. You’ll have to build Z3, and place the binary somewhere on the PATH. After this is done, and assuming your .cabal/bin directory is also on the PATH, testing your source file is a simple as:

liquid [FILE].hs

Let’s Have A Look

Create a haskell source file that contains the following:

headInTail :: [a] -> [a] headInTail l = (tail l) ++ [head l]

After that’s done, let Liquid Haskell analyze your file:

liquid [YOUR_FILE].hs

A bunch of stuff should scroll by, then in a second you’ll see something similar to the following:

**** UNSAFE ********************************************* Doop.hs:5:22: Error: Liquid Type Mismatch Inferred type VV : [a] | VV == l && len VV >= 0 not a subtype of Required type VV : [a] | len VV > 0 In Context VV : [a] | VV == l && len VV >= 0 l : [a] | len l >= 0

If you go to the line and column indicated, you’ll find the argument to tail. Conveniently, it seems that Liquid Haskell comes pre-loaded with definitions for some library functions. Normally, you’ll have to define those yourself. In fact, let’s do just that.

The next logical step here is to write a specification for our function. This specification is a statement about what sorts of values the function can take. Add the following to your source file, in the line above the signature for headInTail:

{-@ headInTail :: {l:[a] | len l > 0} -> [a] @-}

If you re-run liquid on your source file, you’ll see that the warning went away, and the program now indicates that your source is “SAFE”. So, what does this refinement mean?

Basically, these refinements are machine-checked comments. They have no impact on the program, they exist for Liquid Haskell. Think of it as being like an enhanced type signature. Like a normal type signature, we start with the name of the function, then two colons. This part, however:

{l:[a] | len l > 0}

…should be new. Basically, this part says that the list should not be empty. You should read it as “l is a [a] such that len l is greater than zero.” A lot of the notation used by Liquid Haskell comes from formal logic. Let’s break this down some more:


Here we bind a symbol, l, to the first list argument. At any point to the right of this symbol until the end of the scope defined by {}, we can reference this symbol.

{... | ...}

The pipe symbol indicates that we are going to make some statement about the type on the left hand side.

len l > 0

Here we state that the length of l must be greater than 0. It looks like we are calling a function, and we sort of are; len is a measure which is a special function that is used in specifications. However, the subject of measures is a post for another day.

You may now be thinking: “Well this is all well and good, but what’s to stop me from calling this function on an empty list?” To answer that, let’s implement main:

main = do i <- getLine putStrLn $ headInTail i

Add this to your source file, and then run liquid [YOUR_FILE].hs and you’ll notice that Liquid Haskell has a problem with your attempt to call headInTail:

**** UNSAFE ********************************************* Doop.hs:3:29: Error: Liquid Type Mismatch Inferred type VV : [Char] | VV == i && len VV >= 0 not a subtype of Required type VV : [Char] | len VV > 0 In Context VV : [Char] | VV == i && len VV >= 0 i : [Char] | len i >= 0

Liquid Haskell is telling you that it can’t prove that the length of i is greater than 0. If you execute your main function, you should see that it works as expected. Type in a string, and it’ll do the right thing. Push enter right away and you’ll get an exception.

*Main> main hello elloh *Main> main *** Exception: Prelude.tail: empty list

…ick… Let’s fix this:

main = do i <- getLine case i of [] -> putStrLn "Get your head checked." _ -> putStrLn $ headInTail i

Now if you analyze your file, Liquid Haskell should be happy. Honestly, you should be happy too: the tool caught this problem for you. It didn’t go to testing messed up, and the bug certainly didn’t escape testing unfound. Your program is now objectively better:

*Main> main isn't that neat? sn't that neat?i *Main> main Get your head checked.

Again, This Time In Reverse!

Today we’ll be doing Exercise 5 from The C Programming Language. Today’s exercises aren’t terribly challenging, but there is some good stuff in here. The problem is:

Modify the temperature conversion program to print the table in reverse order, that is, from 300 degrees to 0.

I bet you thought we were done with temperature conversion, didn’t you? I’m right there with you, but luckily for us, this is also the section that introduces the for loop, so this is much less tedious. The new program for modification:

#include <stdio.h> int main (int argc, char ** argv) { for (int fahr = 0; fahr <= 300; fahr = fahr + 20) { printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32)); } }

I made a few minor modifications. I gave main the correct signature to keep gcc from complaining, and I also moved the initialization of fahr to the loop. As you may know this is not valid C, and the compiler will throw an error. You might also know that this was made to be valid C in the C 99 revision. To compile this, you just need to add the -std=c99 flag to your gcc call.

To complete this exercise, you only need to modify the loop declaration. Change this line…

for (int fahr = 0; fahr <= 300; fahr = fahr + 20)

…to this…

for (int fahr = 300; fahr >= 0; fahr = fahr - 20)

Pretty straight forward stuff here. fahr starts at 300 instead of 0, and the loop continues so long as it remains greater than 0. Instead of adding 20 each iteration, we subtract it. This can be compiled like so:

gcc -std=c99 -Wall ex5.c -o ex5

In Haskell

Unfortunately, the authors of The C Programming Language did not see fit to include a starting Haskell program for us to modify. This seems like a pretty serious oversight to me, but we’ll just have to make due. Luckily for us, we can use the temperature conversion program I wrote for the last entry in this series. This program handles conversions, and we can use it to produce a table of all conversions between 0 and 300 degrees.

While I’ve established that Haskell does in fact have loops, we won’t be using them here. Instead we’ll be using ranges, functors, and sequencing to solve this problem in a more functional way.

First, we need a list of every 20th number between 0 and 300:

[300.0, 280.0 .. 0.0]

The .. there basically says “You get the idea, do that.” If you put enough information into it so that the compiler can figure out the pattern, and start and end points, the compiler will fill the rest in. In this case, I gave the first two values, telling the compiler I want increments of 20, and I gave it the last value so it knows where to stop.

Unfortunately, as you recall, my conversion program needs members of the Temperature typeclass. Surely I don’t plan to make Double a member, right? Right. We need to map a function over this list to produce a list of Temperatures. But what function should we use?

Something that beginning Haskellers may not realize is that constructors are in fact functions!

Prelude> :t Just Just :: a -> Maybe a Prelude> :t Left Left :: a -> Either a b

…and of course…

*Main> :t Fahrenheit Fahrenheit :: Double -> Fahrenheit

That’s right, we can map Fahrenheit over a functor just like any other function!

map Fahrenheit [300.0, 280.0 .. 0.0]

This will produce a list of every 20th Fahrenheit temperature between 300 and 0. However, we aren’t done yet. We actually need a list of Conversions, because this type is required for our insertConv function. To get this we can map toConversion over this list:

map toConversion $ map Fahrenheit [300.0, 280.0 .. 0.0]

…but that’s starting to get a bit ugly, there must be a better way. Normally, I’m not terribly fond of function composition, in this case it will make our life easier.

Function Composition

Haskell provides an operator . that is used for function composition. Let’s take a look at its type:

Prelude> :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c

The composition operator takes a function that takes some type b as an argument, and that returns some type c. It takes a second function that takes some type a as an argument, and that returns some type b. Finally, it returns some type c.

What does this do for us? Basically, it takes a function a -> b, and a function b -> c, and turns them into a function a -> c. Let’s see an example:

Prelude> :t show show :: Show a => a -> String

The function show takes a member of the Show typeclass and returns a String.

Prelude> :t putStr putStr :: String -> IO ()

The function putStr takes a String, and returns an IO action.

Prelude> :t putStr . show putStr . show :: Show a => a -> IO ()

When we compose the two functions, we get a function that takes a member of the Show typeclass, and returns an IO action.

Logically, calling putStr . show someShowable is the same as calling putStr $ show someShowable or putStr (show someShowable). However, putStr . show is a valid function, where putStr $ show and putStr (show) are compiler errors if you don’t give show an argument.

How does this help us?

*Main> :t Fahrenheit Fahrenheit :: Double -> Fahrenheit *Main> :t toConversion toConversion :: (Temperature a, Temperature b) => a -> (a, b) *Main> :t toConversion . Fahrenheit toConversion . Fahrenheit :: Temperature b => Double -> (Fahrenheit, b)

By composing toConversion and Fahrenheit, we end up with a function that takes a Double as an argument and returns a Conversion from Fahrenheit. We can then map this function over our list of Doubles:

map (toConversion . Fahrenheit) [300.0, 280.0 .. 0.0]

Next, we need to turn these Conversions into TableBuilders. This is a simple matter of composing insertConv:

map (insertConv . toConversion . Fahrenheit) [300.0, 280.0 .. 0.0]

The type of this new function is:

:t (insertConv . toConversion . Fahrenheit) (insertConv . toConversion . Fahrenheit) :: Double -> TableBuilder ()

…or at least it would be if the type inferrer could figure out the type of toConversion. Unfortunately for us, it can’t because the implementation is purposely ambiguous. We need to add a type signature to it:

(insertConv . (toConversion :: Fahrenheit -> (Fahrenheit, Celsius)) . Fahrenheit)


Now we are ready to create our table. This part is actually very easy. Thanks to the library function sequence_ we won’t even need a do block. What does this function do? Let’s look at its signature:

Prelude> :t sequence_ sequence_ :: Monad m => [m a] -> m () Prelude> :t sequence sequence :: Monad m => [m a] -> m [a]

There are two variations of this function. The first, sequence_ evaluates each monadic action, and ignores the results. Basically, suppose we have some function: func :: a -> Monad b

let mList = map func ["foo", "bar", "baz"] in sequence_ mList

…is equivalent to…

do func "foo" func "bar" func "baz" return ()

The second variation evaluates each monadic action, and returns a list of all the results. Suppose we have our same function: func :: a -> Monad b

let mList = map func ["foo", "bar", "baz"] in sequence mList

…is equivalent to…

do foo <- func "foo" bar <- func "bar" baz <- func "baz" return [foo, bar, baz]

If you have a list of monads, you can use sequence or sequence_ to have them all evaluated. Use sequence if you care about the resulting values, use sequence_ if you only care about the monad’s side effect. Which do we want? Let’s look at the signature of insertConv:

*Main> :t insertConv insertConv :: (Temperature a, Temperature b) => Conversion a b -> TableBuilder ()

If we were to use sequence, we’d get a resulting value with the type TableBuilder [()]. Since nobody has any use for a list of empty tuples, we’ll be using sequence_.

So, what does our main look like?

main = do temps <- return (map (insertConv . (toConversion :: Fahrenheit -> (Fahrenheit, Celsius)) . Fahrenheit) [300.0, 280.0 .. 0.0]) putStrLn $ prettyPrint $ buildTable $ do sequence_ temps

This produces the following output:

------------------------------ || Fahrenheit || Celsius || ------------------------------ || 300.0 |> 148.9 || || 280.0 |> 137.8 || || 260.0 |> 126.7 || || 240.0 |> 115.6 || || 220.0 |> 104.4 || || 200.0 |> 93.3 || || 180.0 |> 82.2 || || 160.0 |> 71.1 || || 140.0 |> 60.0 || || 120.0 |> 48.9 || || 100.0 |> 37.8 || || 80.0 |> 26.7 || || 60.0 |> 15.6 || || 40.0 |> 4.4 || || 20.0 |> -6.7 || || 0.0 |> -17.8 || ------------------------------

K&R Challenge 3 and 4: Fiddling With Formatting Strings

Today I’ll be doing things a bit differently. Since exercise 3 and 4 are quite trivial, and because the Haskell implementation will differ significantly from the C solution, I’m going to break them up into separate posts. In this post, I will implement the C solution to problems 3 and 4, and another day I’ll implement the Haskell solution.

The problem for exercise 3 is:

Modify the temperature conversion program to print a heading above the table

The program in question is as follows:

int main (int argc, char ** argv) { float fahr, celsius; int lower, upper, step; lower = 0; upper = 300; step = 20; fahr = lower; while (fahr <= upper) { celsius = (5.0/9.0) * (fahr-32.0); printf("%3.0f %6.1f\n", fahr, celsius); fahr = fahr + step; } return 0; }

If compiled and executed, this will print the following:

0 -17.8 20 -6.7 40 4.4 ... 280 137.8 300 148.9

Strictly speaking, this is a trivial problem. The solution is to add the following line before the while loop:

printf("F C\n");

Now, we have our header:

F C 0 -17.8 20 -6.7 ... 280 137.8 300 148.9

Problem 4 is as follows:

Write a program to print the corresponding Celsius to Fahrenheit table.

This can be implemented by simply changing the line…

celsius = (5.0/9.0) * (fahr-32.0)


fahr = (9.0/5.0) * celsius+32;

…and changing the corresponding printf calls to swap the pairs. Nothing groundbreaking here. Now, let’s talk about my combined solution.

The Solutions

First, we tackle exercise 3. Technically, my previous answer fit the requirements, but I wouldn’t call it “good”. Clearly some pretty printing is in order. Additionally, I’d like to make a table with four columns to print the Fahrenheit to Celsius conversion next to the Celsius to Fahrenheit conversion. To that end, let’s add the following above the loop:

printf("Celsius <- Fahrenheit | Celsius -> Fahrenheit\n"); printf("----------------------|----------------------\n");

This is pretty self-explanatory. We Make some nice pretty columns with a horizontal line between the header and the data. On the left is the Fahrenheit to Celsius conversion, and the right is the Celsius to Fahrenheit conversion. Next, the table.

We’ll need to add extra variables to store the exercise 3 and 4 temperatures. Let’s modify our variables:

float fahr_ex3, celsius_ex3, fahr_ex4, celsius_ex4;

Now we need to calculate both conversions given the same interval:

celsius_ex3 = (5.0/9.0) * (fahr_ex3-32.0); fahr_ex4 = (9.0/5.0) * celsius_ex4+32;

After that’s done, we need to print the table row we just calculated:

printf("%7.1f <- %-10.1f | %7.1f -> %-10.1f\n", celsius_ex3, fahr_ex3, celsius_ex4, fahr_ex4);

Here we’ve updated our printf statement. We now print all four variables on one line. Additionally we’ve updated the formatting options to line the entries up neatly within the columns.

Finally, we need to modify some loop bookkeeping:

fahr_ex3 = lower; celsius_ex4 = lower; while (fahr_ex3 <= upper && celsius_ex4 <= upper) { /*Do Stuff*/ fahr_ex3 = fahr_ex3 + step; celsius_ex4 = celsius_ex4 + step; }

And we’re done! If you compile and execute this new program, the following should print to the terminal:

Celsius <- Fahrenheit | Celsius -> Fahrenheit ----------------------|---------------------- -17.8 <- 0.0 | 0.0 -> 32.0 -6.7 <- 20.0 | 20.0 -> 68.0 4.4 <- 40.0 | 40.0 -> 104.0 15.6 <- 60.0 | 60.0 -> 140.0 26.7 <- 80.0 | 80.0 -> 176.0 37.8 <- 100.0 | 100.0 -> 212.0 48.9 <- 120.0 | 120.0 -> 248.0 60.0 <- 140.0 | 140.0 -> 284.0 71.1 <- 160.0 | 160.0 -> 320.0 82.2 <- 180.0 | 180.0 -> 356.0 93.3 <- 200.0 | 200.0 -> 392.0 104.4 <- 220.0 | 220.0 -> 428.0 115.6 <- 240.0 | 240.0 -> 464.0 126.7 <- 260.0 | 260.0 -> 500.0 137.8 <- 280.0 | 280.0 -> 536.0 148.9 <- 300.0 | 300.0 -> 572.0

Aside from just looking nicer, I’d say this is more in line with the spirit of the exercise. After all, when faced with a requirements document, you can either give a customer exactly what they asked for, or you can give them what they actually want. Which is more likely to get sent back for rework?

The complete source for this solution:

#include <stdio.h> int main (int argc, char ** argv) { float fahr_ex3, celsius_ex3, fahr_ex4, celsius_ex4; int lower, upper, step; lower = 0; upper = 300; step = 20; fahr_ex3 = lower; celsius_ex4 = lower; printf("Celsius <- Fahrenheit | Celsius -> Fahrenheit\n"); printf("----------------------|----------------------\n"); while (fahr_ex3 <= upper && celsius_ex4 <= upper) { celsius_ex3 = (5.0/9.0) * (fahr_ex3-32.0); fahr_ex4 = (9.0/5.0) * celsius_ex4+32; printf("%7.1f <- %-10.1f | %7.1f -> %-10.1f\n", celsius_ex3, fahr_ex3, celsius_ex4, fahr_ex4); fahr_ex3 = fahr_ex3 + step; celsius_ex4 = celsius_ex4 + step; } return 0; }

UPDATE: The Haskell solution can be found here.

DMP Photo Booth 1.0

Well, the day has come and gone. DMP Photo Booth’s final test on June 21st went off without issue, and DMP Photo Booth has left Beta and is now considered “production ready”. The initial 1.0 release can be found on GitHub.

The significance of June 21st is the very reason DMP Photo Booth was created; the 21st is the day of my wedding. My wife wanted a photo booth for the reception. We looked into renting a photo booth, but it turns out that they run around $1,000. I turned to open source. Some quick googling turned up some options, but they were all personal projects or out of date. Sure I could get somebody else’s project working, but what’s the fun in that? I decided that we didn’t need to rent one, or download one, I could build it!

In late 2013, I set to work in earnest. I had a couple of months of downtime in school, and since I’m not currently working it was the perfect time. I decided I had three main objectives for this project: get some arduino experience, get some GTK+ experience, and do this all as portably as possible. I had initially decided to mostly ignore GLib and focus on GTK, but slowly I grew to appreciate GLib for what it is: the standard library that C never had. First I used GModule to handle shared libraries in a portable manner. Next I decided to use GLib primitives to keep from having to deal with cross-platform type wonkiness. Next, having grown tired of dealing with return codes, I refactored the project to use GLib’s exception replacement: GError.

Lessons Learned

It’s not all roses and puppies though. There are certainly things I’d do differently. DMP Photo Booth is developed in an Object Oriented style, passing opaque structs with “method” functions that operate on them. Each component of the program are organized into their own source file with file scoped globals scattered throughout. Said globals are protected by mutexes to create a semblance of thread safety. That said, threading issues have been a major thorn in my side. Long story short: I regret this design choice. While I still feel that this is the correct way to structure C code, and that if globals are required, this is the correct way to handle them; I feel that I should have made more of an effort to limit side effects. Recently, I’ve spent some time doing functional programming, and if I could do it again I’d try to write in a more functional style. Fortunately for me, this is something that a little refactoring could help with.

Additionally, one thing I thought would be a major help is something that began to be a major thorn in my side: NetBeans. As the size of the project grew, NetBeans got slower and slower. It seemed that I spent more time fiddling with IDE settings than actually coding. Even worse is that the IDE-generated makefile is so convoluted that it’s extremely difficult to modify by hand in a satisfying way. I’ve always coded with and IDE so I wouldn’t have even considered not using one, but then I spent some time with Haskell. One of Haskell’s “problems” is that it doesn’t have good IDE support. It doesn’t seem like any IDE really handles it well, so most people use Emacs. Personally, I haven’t really warmed up to Emacs, but GEdit has syntax highlighting for Haskell and a built-in terminal for GHCI. GEdit also has syntax highlighting for C. Next time, I will seriously consider using a lighter-weight text editor for a C project. All this said, I think NetBeans for Java remains the way to go.

What’s Next

Like any program, version 1.0 is just one of many versions. There certainly remains a lot of work to do with DMP Photo Booth. Some major items you are likely to see whenever I get around to working on DMP Photo Booth some more:

Options Dialog

I think anybody who has seen it will agree: the options dialog in DMP Photo Booth is bad. It’s poorly organized, and kind of wonky. Personally, I modify settings using the .rc file, which is telling. This is certainly a high-priority improvement.

Functional Refactor

Like I said above, the code could use a pass to limit side effects. Funtions need to have their side effects limited, and globals need to be eliminated unless absolutely necessary. However, C is not a functional language. While one could argue that function pointers enable functional programming in C, this is a very pedantic argument. I won’t be going crazy with functional programming techniques. There will be no Monads, or for loops being turned into mappings of function pointers.

Optional Module API

An idea I’ve had on the back burner for a while is an optional module API. This would be used for very specific quality-of-life things. For instance, a module could provide a GTK widget to be shown in the options dialog. Any module that doesn’t want to implement any or all of the optional API can just ignore it. The module loading function will gracefully handle the dlsym failure, just treating it as it is: declining to implement the API. I have no plans to change the current existing API, so all you module developers can rest easy!

User Interface Module

It occurred to me that it might be good to have a UI module. This would provide the UI, and wouldn’t be tied to the trigger/printer/camera module start/stop system. This module would be loaded at startup and unloaded on shutdown. This would allow the Photo Booth to use different widget toolkits: QT, Curses, Cocoa, WinForms, or whatever else. Under this scheme, the current GTK+ interface would be abstracted into the reference UI Module.

Things That Would Have Been Nice To Know: Happstack Requests

Stop me if you’ve heard this one before; after trying and failing to find an existing way to some task, a programmer gives up and creates their own method. No sooner than they’ve finished, they find the existing method. This happened to me.

Flash back a week. I’m sitting here working on my on-again-off-again project for a web app to access my home server. I’m trying to implement an image gallery application, and I need a method to read the URI and return images.

Happstack, my framework of choice, provides two methods for routing. There is some template-haskell magic that I don’t care to deal with, and there is string routing combinators. These combinators look like this:

routeFn = dir "foo" $ dir "bar" $ fooBarRouteFn

Basically, routeFn tests to see if the requested path is /foo/bar, and if so, calls the fooBarRouteFn route function. This is all fine and good for simple static routes, but I want to be able to request an arbitrary path on my server, and this solution won’t work. Looking through the Happstack documentation, I find a function called uriRest that “grab[s] the rest of the URL (dirs + query) and passes it to your handler”. Using this, if I request:


… and I call uriRest, I’ll get:


I can then use Parsec to parse that into a usable data structure. Less than ideal, but something I can work with. So, work with it I did, implementing a parser to turn that into:

data URITail = URITail {getRouteDirs :: [String], getQueries :: Maybe [(String, Maybe String)] } deriving (Show)

This worked, though I wouldn’t call it “elegant”. I couldn’t shake the feeling I was doing it wrong. It turns out that I was.

Just Use askRq, Dummy!

It turns out that in any ServerPart, you can call askRq. This returns the Request. Request has a bunch of useful functions, here are a few of particular interest:


rqPaths returns a list of path segments. Exactly what would have been in my URITail‘s getRouteDirs field.


rqInputsQuery returns a list of key-value pairs for each part of the QUERY_STRING. This is exactly what my URITail‘s getQueries field was for. Even better is the fact that this function handles all sorts of encoding issues that I, a web developer with hours of experience, wasn’t even aware of!

and others

Aside from these big two, there are functions to get the request method, get the requesting host, and all sorts of potentially useful information.

Don’t I Feel Silly?

Yeah, but it wouldn’t be the first time. I was pretty proud of my URITail module too, unfortunately it’ll have to languish in the dust bin, serving only as a learning experience. Luckily for you, you can learn from my mistakes.

If you want to route in a dynamic manner in Happstack, feel free to call askRq, and examine the request. Don’t re-invent the wheel.

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