Showing posts from 2016

TMTOWTDI, Haskell Style

I assure you there will be no further allusions to Korean earworms. That said, on to the subject at hand...

Remember the exercise in the online Haskell course that had several tests to filter out weak passwords, all of which had to pass for the fictitious system to allow a String value to be used as a password? I wanted to make it easy to change, so I wanted to take a [String -> Bool] and get a [Bool] I could apply and to for the final thumbs up/thumbs down decision.

The first step: roll my own, which has a pleasing symmetry with map if you write it as a list comprehension:

wonkyMap fs x = [f x | f <- fs]

Then I stumbled across Derek Wyatt's blog post about using sequence for the purpose. Life was good... and then I got Haskell Programming from first principles, and life got better, because its authors do a very good job of explaining the Applicative type class. Applicative defines two functions, pure and (<*>):

class Functor f => Applicative f where
    pure  :: a -&g…

Haskell Tool Stack for Ubuntu 16.04

I've upgraded to Ubuntu 16.04 beta 2, and so far, as I've read elsewhere, it's working very nicely.

(One warning: if you have apcupsd installed, turn it off before upgrading! During the upgrade, something happens that causes the UPS battery to look fully discharged, and apcupsd will obligingly shut your system down... in mid-upgrade. Others have seen this when upgrading to 15.10.)

Now I'm reinstalling the various packages I had previously installed, but for FP Complete's Haskell Tool Stack, trying the Ubuntu instructions FP Complete gives (mutatis mutandis for the version) doesn't work, at least as I write. It looks like it's made its way into the Ubuntu repositories, so that sudo apt-get install haskell-stack will do the trick. I'm also quite pleased to see that 16.04 is actually up to date with respect to GHC. ghc --version shows it has 7.10.3. (Now to look to see whether it's up to snuff on the libghc* packages in the repositories...)

A rather long blast from the past about recursion elimination and a bit of complexity theory

While going through old papers, I found something I wrote as a follow-up to an article by Aaron Banerjee in the February 1999 issue of the world of 68’ micros (you can find a somewhat mangled version online here). Both concern recursion elimination, with the well-known “eight queens” problem (place eight queens on a chess board so that none attacks any other) as an example.
I don’t remember whether I submitted it, or whether it was printed. I’m also embarrassed by the barely legible layout—large Helvetica with nearly no leading. To let me toss the printout and keep the text, and to make it easier to read (if only by me), here it is again, with formatting help from LibreOffice and editing for clarity and concision, not to mention fixing bugs I caused by not directly copying and pasting source code. I will come back and fix indentation on the BASIC09 code, after a little experimentation.
Recursion Elimination and the Eight Queens Problem
In the February 1999 the world of 68’ micros, Aaro…