Wim 🅾 @wim_v12e@octodon.social

I finished a new large #watercolour #drawing. It's probably the most detailed drawing I ever made.

The main figure in the drawing is based on a picture I took at the Gion Matsuri in #Kyoto some years ago. It is a dancer performing the old Japanese court dance "Ran-Ryo-O".

I used Derwent Inktense and Conté Aquaralle pencils as well as watercolour paint (various brands). The paper is Arches 56cmx76cm 300g/m2 rough watercolour paper.

#art #mastoart

I finished my #drawing, on the last day of my holidays! It's the first time I've done such a large #watercolour (50cmx70cm). It's based on a picture I took of a lotus in a pond in a temple (萬福寺) near #Kyoto. #art #mastoartDetail of the lotus pond watercolour

#introductions

I'm Wim, in Glasgow (Scotland).
I like #photography, #reading, #art, #veggie cooking and #japan (#日本語).

I do research on #compilers, #languages and runtimes (e.g. #OpenCL) for #FPGAs, #GPUs and #manycores.

I program in #Perl, #Haskell, #Fortran, #C++,#Python.

@wim_v12e is my main account, I use it mostly for the non-technical stuff. The computer stuff usually goes on @wim_v12e.

I finished my #drawing. It's based on a picture of a scene from the Gion Matsuri that I took in Kyoto last summer. I used #watercolour pencils, acrylic watercolour paint and bistre. #art

To be perfectly honest, I'm only doing all this so I can re-implement it in #rakulang. Turns out it is really easy too:

role BoolBB[\b] {
method unBoolBB(\t,\f) {
b.(t,f);
}
}

and because Raku is gradually typed, the polymorphism is of course for free.

Now you probably wonder, what good is all this anyway? Yu're thinking, "I'm a very busy man, I have other cases besides this one."
There are many uses for this encoding but what we are using it for is to compose interpreters. Essentially, if your algebraic data type represents an abstract syntax tree, then the BB encoding of it is a universal interpreter. Every particular implementation of the function is an interpreter, and they are composable.

#haskell

We can also encode product types in this way:

newtype PairBB t1 t2 =
PairBB {
unPairBB :: forall a .
(t1 -> t2 -> a) -> a
}

fst p = unPairBB p (\x y -> x)
snd p = unPairBB p (\x y -> y)

bbp = PairBB $ \p -> p 42 "forty-two"

That means that the BB encoding provides a way to represent algebraic data types as polymorphic functions.

There are other reasons why the Boehm-Berarducci encoding, but first I'd like to give another example: the encoding of the Boolean type

data Bool = True | False

is

newtype BoolBB =
BoolBB (forall a . a -> a -> a)

So this is a function of two arguments that returns something of the same type. With two selectors very similar to those for Maybe:

true = \t f -> t
false = \t f -> f

we can write

trueBB = BoolBB true
falseBB = BoolBB false

And why do I think this is nice? Because I have encoded a sum type:

data Maybe b = Just b | Nothing

as a single function type which, if anything, is more like a product type. The record type syntax is just to provide a label for easy access to the function.

With that warning, here is my implementation of a Maybe type in Boehm-Berarducci encoding:

newtype MayBB b = MayBB {
unMayBB :: forall a .
(b -> a)
-> a
-> a
}

and I think it's beautiful.

With these little helpers:

just x = \j' n' -> j' x
nothing = \j' n' -> n'

we get the following:

-- Plain Maybe
mb :: Maybe Int
mb = Just 42

mbn :: Maybe Int
mbn = Nothing

-- BB Maybe
mbb:: MayBB Int
mbb = MayBB $ just 42

mbbn :: MayBB Int
mbbn = MayBB nothing

#haskell

"Boehm-Berarducci's paper has many great insights. Alas, the generality of the presentation makes the paper very hard to understand. It has a Zen-like quality: it is incomprehensible unless you already know its results."
(http://okmij.org/ftp/tagless-final/course/Boehm-Berarducci.html)
#haskell

The poet published some poems containing racial slurs. His friends had begged him to reconsider publishing them, his editor had pleaded with him to withdraw them, but he had insisted that they stay. The poems were commenting on prejudice, he pointed out, and not condoning it. They showed how derogatory words cause people to see others in terms of stereotypes rather than as individuals.

(This is the poem Björk used for her song "Sun in My Mouth")

i will wade out
till my thighs are steeped in burning flowers
I will take the sun in my mouth
and leap into the ripe air
Alive
with closed eyes
to dash against darkness
in the sleeping curves of my body
Shall enter fingers of smooth mastery
with chasteness of sea-girls
Will i complete the mystery
of my flesh
I will rise
After a thousand years
lipping
flowers
And set my teeth in the silver of the moon