Prolog exercise: Min and Max of a list

As a fun exercise I implemented a predicate to calculate the minimum and maximum element of a list.

min_and_max([A], A, A).
min_and_max([H|R], N, X):-
    min_and_max(R, RN, RX),
    N is min(H, RN),
    X is max(H, RX).

Next I wrote a version with last-call optimization.

min_and_max_2 …
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One thing I miss from Mercury

It's been a year since I've written any Mercury (the purely logic/functional programming language), but there's one feature I miss. It's nothing fancy, and certainly one of the less interesting features Mercury has to offer.

The Problem

To understand this particular feature let's look at where other languages fail …

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My Experience with Xonsh

Since January I've been using Xonsh as my main shell on both my work and personal machines. In this post I will describe what I liked and what I didn't. TLDR; I liked that I found myself scripting more under Xonsh, but it can be slow and inconvenient as a …

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Constructing tests for legacy code

Every company has legacy code sitting somewhere that isn't well understood. Maybe it was written long ago and the original authors have moved on. Here's a high ROI approach to getting that code tested.

The code I have in mind was written in a time when priorities were different, the …

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Using Goatcounter on NearlyFreeSpeech.net

Here's some quick and dirty notes on setting up GoatCounter on Nearly Free Speech dot net. There are some rather opaque hoops you need to jump through in order to setup the service. I make no claim that this tutorial is perfect, but hopefully it helps someone else who is …

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Hunting for a Link

There you are, browsing the web, maybe using a popular search engine. You enter your query and scan the results for a title that answers your question. There it is! You see it, the third result. You move your mouse to click.

And what's that? what's happening? You've clicked on …

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Python string concatenation

I spent a little bit of time looking into Python string concatenation times. I learned that newer versions of Python implement an optimzation for the plus equals operator. In the future I'd like to know what this optimization is. It seemed simple enough to escape by introducing an intermediate variable …

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Real World Awk: Fastlaning localizations

What is Awk?

Awk is a weird mix of command-line tool and programming language. It's a language specifically designed for text processing. By default it has a notion of rows and separators. Think CSV but without support for quoted separators.

Problem

Fastlane is a tool for automating iOS app development …

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Map a function or predicate in Mercury lang

Mapping a function in Mercury lang is pretty simple. First you need a function:

:- func myfunc(string) = string.

myfunc(In) = Out :- 
  append(In, In, Out).

Then you map it

:- import_module list.

NewList = map(myfunc, OldList)

We can do the same thing with a predicate.

:- pred mypred(string::in, string::out …
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How to convert a predicate from multi to det in Mercury lang?

So you're learning Mercury lang and you have a predicate with multi determinism. Maybe it looks something like this:

:- pred school(string:: in, string:: out) is multi.

fish(Fish, ParticularFish) :-
  ( append("One ", Fish, ParticularFish)
  ; append("Two ", Fish, ParticularFish)
  ; append("Red ", Fish, ParticularFish)
  ; append("Blue ", Fish, ParticularFish)
  ).

Which has four solutions …

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