Implementing pattern-matching in JavaScript (short version)

@FGRibreau
Implementing pattern-matching in JavaScript

@FGRibreau
Implementing pattern-matching in JavaScript
…or how to play with EcmaScript shortcoming

@FGRibreau
Une syntaxe de pattern-matching en JS ?
Sortez l'artillerie lourde.

@FGRibreau
_.flatten(links).map(link => {
[{protocol: 'HTTP'}]: => 1,
[{protocol: 'AMQP'}]: => 2
});
It would it be awesome to use some pattern-matching there right?

@FGRibreau
SyntaxError: /Users/FG/www/iadvize-services-
orchestration-tools/src/api/src/repositoryManagers/
github.js: Unexpected token (185:32)
183 |
184 | _.flatten(links).forEach(link => {
> 185 | [{protocol: 'HTTP'}]: => 1,
| ^
186 | [{protocol: 'AMQP'}]: => 2
187 | });
188 |
... and of course it’s not JS valid syntax

@FGRibreau
syntactically short
javascript-minded syntax
functionally-way
I wanted a pattern-matching library

@FGRibreau
links.map(link => {
});
Ok, so this is not valid,
what precisely is not valid and
how can we make it valid?

@FGRibreau
links.map(link => {
[{protocol: 'HTTP'}]: () => 1,
[{protocol: ‘AMQP'}]: () => 2
});
links.map(link => {
[{protocol: 'HTTP'}]: 1,
[{protocol: ‘AMQP'}]: 2
});
links.map(link => {
});

@FGRibreau
{
[{protocol: ‘HTTP'}]: 1,
[{protocol: ‘AMQP’}]: 2
}
The rest is syntactically valid
ES6 "computed property names"
{
[{protocol: ‘HTTP’}]: () => 1,
[{protocol: ‘AMQP’}]: () => 2
}

@FGRibreau
links.map(link => {})
[undefined, undefined]
links.map(link => {1})
[undeﬁned, undeﬁned]
links.map(link => {return 1})
[1,1]
links.map(link => 1)
[1,1]
Syntactically valid, semantically invalid
… but then I won't have my pattern matching.

@FGRibreau
If I go from there…
});

@FGRibreau
If I go from there…
});
_.flatten(links).map(match({
[{protocol: 'AMQP'}]: 2
}));
…to there…

@FGRibreau
… then it’s syntactically correct!
_.flatten(links).map(match({
}));

@FGRibreau
… would be great too !
const linkNumber = match(link,{
});

@FGRibreau
{
'[object Object]': 2
}
evaluates to
{
}

@FGRibreau
evaluates to
{
[when({protocol: ‘HTTP’})]: 1,
[when({protocol: ‘AMQP'})]: 2
}
{
'{"protocol":"HTTP"}': 1,
'{"protocol":"AMQP"}': 2
}

@FGRibreau
Order is lost by match’s object
.map(match({
[when.range(0, 43)]: 42,
[when(42)]: 72
}))
when.range(0,43) => '["Symbol(match.pattern.RANGE)",0,43]'
when(42) => '[42]'
JS objects are an unordered collection of properties

@FGRibreau
Fixing properties declaration
?
.map(match({
[when.range(0, 43)]: 42,
[when(42)]: 72
}))
.map(match(new Map([
[ when.range(0, 43), 42 ],
[ when(42), 72 ],
[ when(), 'never should be hit' ]
])))
NO!

@FGRibreau
?
.map(match({
[when.range(0, 43)]: 42,
[when(42)]: 72
}))
.map(match([
[ when.range(0, 43), 42 ],
[ when(42), 72 ],
[ when(), 'never should be hit' ]
]))
NO!

@FGRibreau
.map(match({
[when.range(0, 43)]: 42,
[when(42)]: 72
}))
- same callsite
- single thread
- sequential evaluation

@FGRibreau
when.range(0,43) => '[0, "Symbol(match.pattern.RANGE)",0,43]'
when(42) => '[1, 42]'
… #problemSolved …
.map(match({
[when.range(0, 43)]: 42,
[when(42)]: 72
}))
- same callsite
- single thread
- sequential evaluation

@FGRibreau
How to get matched value?
const fact = match({
[when(0)]: 1,
[when()]: n * fact(n-1)
});
fact(10);
n is not deﬁned

@FGRibreau
How to get matched value?
[when(0)]: 1,
[when()]: n * fact(n-1)
});
fact(10);
simple, use a function:
[when(0)]: 1,
[when()]: (n) => n * fact(n-1)
});
fact(10); // 3628800

@FGRibreau
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Implementing pattern-matching in JavaScript (short version)

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