- 2021-11-09: You have:
`\k _v -> f k`

Curry away the arguments. - 2021-11-09: Hello, all. It's been a minute.
Here's a #1Liner #Haskell problem

You have m :: Map a b

You want to filter it by s :: Set a

so that m has keys only in s.

How would you do that?

- O_O @dysinger: let map = Data.Map.fromList [(1, "one"), (2, "two"), (3, "three")]

set = Data.Set.fromList [1,3,5,7,9]

in Data.Map.fromList [ elem | elem <- Data.Map.toList map, Data.Set.member (fst elem) set ] - ephemient @ephemient: Map.filterWithKey (\k _ -> Set.member k set) map
- ephemient @ephemient: Map.intersectionWith const map $ Map.fromDistinctAscList [(k, ()) | k <- Set.toAscList set]
- じょお @gotoki_no_joe Map.intersection m (Map.fromSet (const ()) s)

- O_O @dysinger: let map = Data.Map.fromList [(1, "one"), (2, "two"), (3, "three")]

# Typed Logic

Incorporates strong typing over predicate logic programming, and, conversely, incorporates predicate logic programming into strongly typed functional languages. The style of predicate logic is from Prolog; the strongly typed functional language is Haskell.

## Tuesday, November 9, 2021

### November, 2021 1HaskellADay 1Liners

## Sunday, June 6, 2021

### Why Kleisli Arrows Matter

We're going to take a departure from the style of articles regularly written about the Kleisli category, because, firstly, there aren't articles regularly written about the Kleisli category.

That's a loss for the world. Why? I find the Kleisli category so useful that I'm normally programming in the category, and, conversely, I find most code in industry, unaware of this category, is doing a lot of the work of (unintentionally) setting up this category, only to tear it down before using it effectively.

So. What is the Kleisli Category? Before we can properly talk about this category, and its applications, we need to talk about monads.

**Monads**

A monad, as you can see by following the above link, is a domain with some specific, useful properties. If we have a monad, *T,* we know it comes with an unit function, η, and a join function, μ. What do these 'thingies' do?

*T*, the monadic domain, says that if you are the domain, then that's where you stay:*T*:*T*→*T.*

"So what?" you ask. The beauty of this is that once you've proved you're in a domain, then all computations from that point are guaranteed to be in that domain.

So, for example, if you have a monadic domain called Maybe, then you know that values in this domain are Just some defined value or Nothing. What about null? There is no null in the Maybe-domain, so you never have to prove your value is (or isn't) null, once you're in that monadic domain.

- But how do you prove you're in that domain? η lifts an unit value (or a 'plain old' value) from the ('plain old') domain into the monadic domain. You've just proved you're in the monadic domain, simply by applying η.

η null → fails

η some value → some value in *T.*

- We're going to talk about the join-function, μ, after we define the Kleisli category.

**The Kleisli Category**

Okay. Monads. Great. So what does have to do with the Kleisli category? There's a problem with the above description of Monads that I didn't address. You can take any function, say:

*f : A *→ *B*

and lift that into the monadic domain:

*f ^{T} : T A *→

*T B*

... but what is *f ^{T}*?

*f*looks exactly like

^{T }*f,*when you apply elimination of the monadic domain,

*T.*How can we prove or 'know' that anywhere in our computation we indeed end up in the monadic domain, so that the next step in the computation we know we are in that monadic domain, and we don't have to go all the way back to the beginning of the proof to verify this, every time?

Simple: that's what the Kleisli category does for us.

What does the Kleisli category do for us? Kleisli defines the Kleisli arrow thusly:

*f ^{K} : A *→

*T B*

That is to say, no matter where we start from in our (possibly interim) computation, we end our computation in the monadic domain. This is fantastic! Because now we no longer need to search back any farther than this function to see (that is: to *prove*) that we are in the monadic domain, and, with that guarantee, we can proceed in the safety of that domain, not having to frontload any nor every computation that follows with preconditions that would be required outside that monadic domain.

null-check simply vanishes in monadic domains that do not allow that (non-)value.

Incidentally, here's an oxymoron: 'NullPointerException' in a language that doesn't have pointers.

Here's another oxymoron: 'null value,' when 'null' means there is no value (of that specified type, or, any type, for that matter). null breaks type-safety, but I get ahead of myself.

Back on point.

So, okay: great. We, using the Kleisli arrows, know at every point in that computation that we are in the monadic domain, *T,* and can chain computations safely in that domain.

But, wait. There's a glaring issue here. Sure, *f ^{K }*gets us into the monadic domain, but let's chain the computation. Let's say we have:

*g ^{K} : B *→

*T C*

... and we wish to chain, or, functionally: compose *f ^{K }*and

*g*? We get this:

^{K}*g ^{K}* ∘

*f*⇒

^{K }*A*→

*T B*→

*TT C*

Whoops! We're no longer in the monadic domain *T, *but at the end of the chained-computation, we're in the monadic domain *TT*, and what, even, is that? I'm not going to answer that question, because 1) who cares? because 2) where we really want to be is in the domain *T, *so the real question is: how do we get rid of that extra *T* in the monadic domain *TT *and get back into the less-cumbersome monadic domain we understand, *T*?

- That's where the join-function, μ, comes in.

*TT A*→

*T A*

*T,*states that when you join a monad of type

*T*to a monad of that same type, the result,

*TT,*when joined, simplifies to that (original) monad,

*T.*

*g*∘

^{K}

^{K}*f*⇒

^{K }*A*→

*T B*→

*T C*

**Practical Application**

*dependently-optional.*The solution the engineering team at that time took was to store every field of every row of the 2000 fields of the mortgage appraisal form.

*automagically*skipped for absent values. Only values

*present*were stored. Only computations on

*present*values were performed. The Sales Comparison Approach had over 600 fields, all of them optional, many of them

*dependently-optional,*and only the values present in those 600 fields were stored. The savings in data storage was

*exponentially*more efficient for the Sales Comparison Approach section as compared to the storage for the rest of the mortgage appraisal.

*langua fraca*for that project) required I first implement the concept of both Function and Functor, using inner classes, then I needed to implement the concept of Monad, then Maybe, then, with monad, I needed to implement the Kleisli Arrow monadic-bind function to be able to stitch together dozens and hundreds of computations together, without one single explicit null-check.

**Summary**

## Tuesday, June 1, 2021

### June 2021 1HaskellADay Problems and Solutions

- Today's #haskell problem: what is 'today' in a data-set. Interesting question. In today's #haskell solution, we build a SQL query, ... WITH HASKELL!

## Saturday, May 8, 2021

### May 2021 1HaskellADay 1Liners: problems and solutions

- 2021-05-24, Monday:

Map.partitionWithKey's discriminator is

`p :: k -> a -> Bool`

But I have a function that discriminates only on the key:

`part :: k -> Bool`

write a function that translates my discriminator that can be used by Map.partitionWithKey:

`g :: (k -> Bool) -> (k -> a -> Bool)`

- Social Justice Cleric @noaheasterly:

`g = (const .)`

- Social Justice Cleric @noaheasterly:
- 2021-05-09, Sunday:

**THE SEQUEL!**Okay, kinda the same, ... but not:

You have:

`f :: m a`

You want:`g :: b -> m b`

Where

`g`

runs`f`

, but accepts an argument,`b`

.

`g`

drops the result of`f`

(... on the floor? idk)

`g`

returns the argument`b`

lifted to the domain,`m`

GO!

- Denis Stoyanov Ant @xgrommx:

`g (phantom . pure)`

This is just joke) - Social Justice Cleric @noaheasterly:
`(f $>)`

- Denis Stoyanov Ant @xgrommx:
- 2021-05-08, Saturday: two-parter
- You have
`f :: a -> m b`

You want`g :: a -> m a`

That is to say:

`g`

is a function that returns the input and drops the output of`f`

.so:

`blah :: (a -> m b) -> (a -> m a)`

- What is a gooder name for the
`blah`

-function?

- Jonathan Cast #AJAA #Resist @jonathanccast:

`returnArg = (*>) <$> f <*> return`

- Social Justice Cleric @noaheasterly:

`liftA2 (<*)`

- You have

## Tuesday, April 6, 2021

### April 2021 1HaskellADay 1Liners Problems and Solutions

- 2021-04-20, Tuesday:
So, I had this problem

I have

`pairs :: [(a, IO [b])]`

but I want

`pairs' :: IO [(a, b)]`

`sequence`

a gives me something like I don't know what: distributing the list monad, not the IO monad. Implement:`sequence' :: [(a, IO [b])] -> IO [(a, b)]`

- p h z @phaazon_:
`fmap join . traverse (\(a, io) -> fmap (map (a,)) io)`

- lucas卞dicioccio, PhD @lucasdicioccio:
Just to annoy you I'll use the list-comprehension syntax you dislike.

`solution xs = sequence [fmap (a,) io | (a, io) <- xs]`

- Benkio @benkio89:

`fmap concat . traverse (\(a,iobs) -> fmap (a,) <$> iobs)`

- Social Justice Cleric @noaheasterly

`fmap concat . traverse (getCompose . traverse Compse)`

- Social Justice Cleric @noaheasterly

`fmap (concatMap getCompose) . getCompose . traverse Compose. Compose`

- Basile Henry @basile_henry: Slightly less polymorphic:

`sequence' = traverse @[] (sequence @((,) _) @IO)`

- Basile Henry @basile_henry: I think it's just

`traverse sequence`

;) - Jérôme Avoustin @JeromeAvoustin: there surely is a shorter version, but I could come up with...

`fmap join . sequence . (fmap . fmap) sequence . fmap sequence`

- p h z @phaazon_:
- 2021-04-16, Friday:
You have a monad, or applicative, and you wish to execute the action of the latter but return the result of the former. The simplest representation for me is:

`pass :: IO a -> b -> IO b`

so:

`return 5 >>= pass (putStrLn "Hi, there!")`

would return

`IO 5`

GO!

- D Oisín Kidney @oisdk
`flip (<$)`

- ⓘ_jack @Iceland_jack
`($>)`

- D Oisín Kidney @oisdk
- 2021-04-12, Monday:
A function that takes the result of another function then uses that result and the original pair of arguments to compute the result:

f :: a -> a -> b g :: b -> a -> a -> c

so:

(\x y -> g (f x y) x y)

curry away the x and y arguments.

- 2021-04-07, Wednesday:

you have`(Maybe a, Maybe b)`

you want`Maybe (a, b)`

If either (Maybe a) or (Maybe b) is

`Nothing`

then the answer is`Nothing`

.If both (Maybe a) and (Maybe b) are (Just ...)

then the answer is`Just (a, b)`

WHAT SAY YOU?

- Jérôme Avoustin @JeromeAvoustin:
`bisequence`

- p h z @phaazon_ with base:
`uncurry $ liftA2 (,)`

- greg nwosu @buddhistfist:
`(,) <$> ma <*> mb`

- Jérôme Avoustin @JeromeAvoustin:

## Thursday, April 1, 2021

### April 2021 1HaskellADay Problems and Solutions

- 2021-04-28: We build a recommendation-system from spotty data for today's #haskell problem. runRecs rekt the run of the ... rektrun? Bah: today's #haskell solution, with implication to the rescue!
- 2021-04-26: Today's #haskell problem, on m̸͈̣̈́̉̉̊̈́y̴̢͌̇̕ ̷̡̗͎̰̠͚̓̈́͠B̷̙͋͊̈̕͘Į̷̻̱͔̼̙͖͚̥̄͝R̴͉̖̻̬̗͕͋̽̉̚͝T̵͈̮͙͚̤̘̦̮͐̓́̔̂͑̋̐̐̄H̶̬̳͔͔̳̀̉̇̋͒D̵̢̡̧̦̖̝͖͕͕̤̊Ȃ̷̧̩̼͔̱͙̟̺̕Y̴̡̢̮̘͛̄̽͛̆̃͌ we solve what every Gubmint agency faces every hour of the day: what does this 'TLA' stand for? Today's #haskell solution: Don't mind me. I'm just TLA'ing over here.
- 2021-04-23: WHO WOULD WIN THIS FIGHT: Ragnarr Loðbrók or Miyamoto Musashi? Today's #haskell problem gives a bean-counter's approach, ... LITERALLY! Today's #haskell solutions shows that the winner of the Viking-Samurai deth-match is: ViewPatterns! ... obviously. 😎
- 2021-04-22: In today's #haskell exercise we try to find the opposite of Kung Fu Fighting, and we fail (because there is no opposite of awesome ... besides fail. GEDDIT?) So we try to find the dual of Kung Fu Fighting. Easy, no? "Your WuShu Praying Mantis-style is strong, but can it match my Tiger Claw-style?" Narrator: "We find out the thrilling conclusion in today's #haskell solution!"
- 2021-04-14: In today's #haskell exercise we learn it requires 'Expert Timing' to be 'Kung Fu Fighting.' So, we have fighting; we have noodles. We have everything we need for today's #haskell solution.
- 2021-04-12: For today's #haskell problem we are lifting a "plain" method into the monadic domain ... yes, I'm influenced by @BartoszMilewski writing. Why do you ask? Today's #haskell solution: Function: lifted.
- 2021-04-09: We get all officious, ... sry: 'OFFICIAL' ... with today's #haskell problem. Today's #haskell solution shows us that you haven't met real evil until your TPS-report cover letter is ... Lorem Ipsum.
- 2021-04-08, Thursday: In today's #haskell problem we optionally parse a string into some value of some type. Some how. Today's #haskell solution shows us the sum of the numbers in the string was 42 (... no, it wasn't).
- 2021-04-07, Wednesday: decode vs. eitherDecode from Data.Aeson is today's #haskell exercise. In today's #haskell solution we find that it's nice when eitherDecode tells you why it failed to parse JSON.
- 2021-04-05, Monday: Today's #haskell exercise is to make a vector that is Foldable and Traversable. Today's #haskell solution gives us Vectors as Foldable and Traversable. YUS!
- 2021-04-01, Holy Thursday: In today's #haskell exercise we learn the WHOPR beat the Big Mac. We also learn how to make a safe call to an HTTPS endpoint, but that's not important as the WHOPR beating the Big Mac. "Would you like to play a game?" Today's #haskell solution says: "Yes, (safely, please)."

## Tuesday, March 23, 2021

### March 2021 1HaskellADay 1Liners

- 2021-03-23:
You have

`[a]`

and`(a -> IO b)`

.You want

`IO [(a, b)]`

That is, you want to pair your inputs to their outputs for further processing in the IO-domain.

- Chris Martin @chris__martin:
`\as f -> traverse @ [] @ IO (\a -> f a >>= \b ->`

`return (a, b)) as`

- cλementd Children crossing @clementd:
`traverse (sequenceA . id &&& f)`

(actually,

`tranverse (sequence . (id &&& f))`

)Or

`p traverse (traverse f . dup)`

- Chris Martin @chris__martin:
- 2021-03-23: You have
`[([a], [b])]`

You want

`([a], [b])`

so:

`[([1,2], ["hi"]), ([3,4], ["bye"])]`

becomes

`([1,2,3,4],["hi","bye"])`

- karakfa @karakfa:
`conc xs = (concatMap fst xs, concatMap snd xs)`

- karakfa @karakfa: