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# Package "pha"

Title: Probabilistic Horn Abduction Not rated. Create the first rating! 0.1.2 13cb25344545315366ca03e132d44f29a339275c Samer Abdallah https://github.com/samer--/pha.git dcgutils typedef

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0.1.1baca1cc93a17aa37d8811da25dfc43f49fd62d231https://github.com/samer--/pha.git
0.1.213cb25344545315366ca03e132d44f29a339275c6https://github.com/samer--/pha.git

# Probabilistic Horn Abduction

### Reimplemented by Samer Abdallah

This is an implementation of probabilistic Horn abduction (pHa), a form of probabilistic programming consisting of an object language that includes random variables and random world semantics, and an inference strategy based on a simulated prioritised multithreaded search for solutions of probabilistic goals.

To run a test script from the shell:,

`\$ swipl -s <path to pha>/scripts/run.pl`

Alternatively, you can run it from the SWI command line with:

`?- [pack(pha/scripts/run)].`

This will load the alarm.pha model and drop you immediately into the interactive stateful top level. The state consists of observations that have been made, and is initially empty. You can type ``help.`` to get a list of commands. To get the probability of a goal, try, eg

`>> prob(fire(yes),P).`

This will tell you the probability that the building is on fire. To test the old adage that 'where there's smoke there's fire', you can check the probabilities:

```>> prob(fire(yes)|smoke(yes), P).
>> prob(fire(no)|smoke(yes), P).```

So there you are: where there's smoke there may or may not be fire. To push an observation on to the stack, do, eg

`>> observe(alarm(yes)).`

This means that you have observed that the fire alarm is on. What is the probability that the building is on fire now?

`>> prob(fire(yes),P).`

What is the probability that the building is on fire if, in addition to the alarm being on, there is also smoke?

`>> prob(fire(yes)|smoke(yes),P).`

What are the explanations for the alarm being on?

`>> explanations(0). % 0 means account for ALL probability mass`

Or to get them one by one,

`>> explanation(E).`

There are more models in `pack(pha/models)`. The run.pl script adds this path as the file search path 'pha'.

### Implementation notes

This implementation is a complete re-write of Poole's which I did in 2015, removing the use of mutable state and failure driven processing in the core interpreter. The original used mutable state to manage a collection of prioritised 'threads' (see below), whereas this one uses state threading and DCG notation to that instead. It also uses lazy lists to manage the generation of explanations on demand, and uses another layer of DCG state threading to keep track of a stack of observations, as illustrated above.

Given more recent work on probabilistic programming, such as Oleg Kiselyov's Hansei, Church, Bher, WebPPL, Anglican and so on, it is interesting to see PHA and Poole's original implementation in terms of a multithreaded exploration of probabilistic choices, with a meta-interpreter that allows the current continuation to be captured whenever a random variable is to be sampled (cf. delimited continuations in Hansei or CPS-transform in WebPPL), a per-thread state consisting of a record of random variable choices made so far (cf Hansei's lazy and Church's mem), and sort of thread scheduler that runs the threads with highest probability first.

In this light, it might be interesting to re-implement this not as an object language with an interpreter, but in 'direct style' using delimited continuations (shift/reset) to handle the simulated multi-threading.

## Contents of pack "pha"

Pack contains 20 files holding a total of 86.8K bytes.