Patterns

The goal of the pattern DSL is ease of writing patterns. Because we can rely on Clojure's macro system, we don't have to deal with string-based patterns (such as Rubocop's otherwise excellent Node Pattern). Therefore, all of the examples in this page will be using raw Clojure code.

High-level description

The macro noahtheduke.splint.pattern/pattern builds the matching function. It takes a quoted form (either a single literal or a collection with arbitrarily nested data) and returns a function that will either match the given shape or return nil.

Types

Inside of a pattern, each element has roughly the same "type" as in Clojure, using noahtheduke.splint.pattern/simple-type. The table below shows an example of the input, the keyword returned by simple-type, the comparison function created by pattern, and any additional information.

Example	simple-type	Comparison function	Notes
nil	:nil	(nil? form)
true / false	:boolean	(identical? given form)
\c	:char	(identical? given form)
1 / 1N / 1.0 / 0x1	:number	(= given form)	Debatable whether to treat numbers as different. So far, patterns haven't needed both, but when they do we can split them up.
:foo / :foo/bar	:keyword	(identical? given form)	Because this isn't a string parsed by edamame, auto-resolved keywords must be handled in special cases. See below for more details.
"foo"	:string	(.equals ^String given form)	Interop is fastest.
'foo / foo	:symbol	(= given form)	Symbols don't support identical?. Some symbols are treated differently as part of the Pattern DSL. See below for more details.
(:a 1)	:list	(seq? form)	If there's a variable-length pattern, size is checked with <=.
[:a 1]	:vector	(vector? form)	If there's a variable-length pattern, size is checked with <=.
{:a 1}	:map	(map? form)	We only support matching on "simple" keys (no collections), so the size is checked with <=.
#{:a 1}	:set	(set? form)

Special patterns

Some symbols are treated differently as part of the Pattern DSL. These are useful when writing more complex patterns.

Special patterns are in the shape of (sym binding opts). Some have shorted forms. Any time there is a binding, if the given binding symbol doesn't start with a question mark, it is changed to have one: (? x) puts ?x in the returned map from pattern.

• ? is for single-value binding: (? x pred?) (short form ?x). Matches anything and binds it to ?x. If a predicate is given, it is called on the matched value and only binds when truthy.
• ?* is for binding zero or more values in a sequence: (?* x pred?) (short form ?*x). Matches any number of items and binds them to ?x. If a predicate is given, it is called on each matched value with (every? pred? items) and only binds when that returns true.
• ?+ is the same as ?* (short form ?+x) but matches one or more items in the sequence.
• ?? is the same as ?* (short form ??x) but matches only zero or one items.
• ?| binds a single value as ?, but requires the third arg to be a vector of simple types, which it will try to match left-to-right: (?| x [a b c]) will only match a, b, or c, but not d. Due to the required vector, there is no short form.

The binding _ or short form ?_ are special: they match anything like a regular binding but don't create bindings in the map returned by pattern. They also don't unify with each other, so [_ 1 _] matches both [0 1 0] and [0 1 2].

All predicates are resolved using clojure.core/requiring-resolve. It first tries to resolve with clojure.core, then with noahtheduke.splint.rules.helpers, then with the current namespace. If it can't resolve to a function, an ExceptionInfo is thrown.

Note: Any of the above can be treated as literals instead of DSL symbols by using the metadata :splint/lit: (quote _) -> :any, (quote ^:splint/lit _) -> :symbol.

Maps

As of 1.0, maps in patterns cannot use complex keys: '{(1 2 3) :true}. This is due to the complexity of implementation. It's possible to implement it, but I haven't found a need yet and implementing collections for sets was already so much work I didn't feel like moving that logic over.

That said, I would love to have this implemented, if anyone wants to take a swing at it. :eyebrows: