Re: Factor

Peter Bengtsson wrote about building a Python dict that can report which keys you did not use:

This can come in handy if you’re working with large Python objects and you want to be certain that you’re either unit testing everything you retrieve or certain that all the data you draw from a database is actually used in a report.

For example, you might have a SELECT fieldX, fieldY, fieldZ FROM ... SQL query, but in the report you only use fieldX, fieldY in your CSV export.

class TrackingDict(dict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self._accessed_keys = set()

    def __getitem__(self, key):
        self._accessed_keys.add(key)
        return super().__getitem__(key)

    @property
    def accessed_keys(self):
        return self._accessed_keys

    @property
    def never_accessed_keys(self):
        return set(self.keys()) - self._accessed_keys

We can build a version of this in Factor intended to show off a few language features. The original version in Python used inheritance versus composition to implement the data structure. Instead, we build a data structure that will wrap an existing assoc and delegate to it.

First, a simple tuple definition that will have the underlying assoc and a set of accessed keys:

TUPLE: tracking-assoc underlying accessed-keys ;

: <tracking-assoc> ( underlying -- tracking-assoc )
    HS{ } clone tracking-assoc boa ;

INSTANCE: tracking-assoc assoc

We then implement the assoc protocol by using delegation to the underlying assoc, with an override for tracking accessed keys:

CONSULT: assoc-protocol tracking-assoc underlying>> ;

M: tracking-assoc at*
    [ underlying>> at* ] [ accessed-keys>> adjoin ] 2bi ;

And for fun – since we could have built a normal word to do this – we define a protocol slot that we then implement to compute the never accessed keys:

SLOT: never-accessed-keys

M: tracking-assoc never-accessed-keys>>
    [ underlying>> keys ] [ accessed-keys>> ] bi diff ;

And we show it works using a simple example from the original blog post:

H{
    { "name" "John Doe" }
    { "age" 30 }
    { "email" "jd@example.com" }
} <tracking-assoc>

"name" over at "John Doe" assert=

[ accessed-keys>> "Accessed keys: %u\n" printf ]
[ never-accessed-keys>> "Never accessed keys: %u\n" printf ] bi

Which prints this:

Accessed keys: HS{ "name" }
Never accessed keys: HS{ "email" "age" }

Fun!