specter-acset

Inline-cached bidirectional navigation for Julia data structures

Version: 1.0.0 Trit: 0 (Ergodic - coordinates navigation)

From Clojure Specter to Julia

Nathan Marz's Specter library for Clojure provides bidirectional data navigation where the same path expression works for both selection AND transformation. This skill ports those patterns to Julia with extensions for S-expressions and ACSets.

Key Insights from Specter Talks

"Rama on Clojure's Terms" (2024)

"comp-navs is fast because it's just object allocation + field sets"

Specter's performance comes from:

1. Inline caching: Paths compiled once, reused at callsite
2. Continuation-passing style: Chains of next_fn calls
3. Navigator protocol: Uniform interface for all data types

"Specter: Powerful and Simple Data Structure Manipulation"

"Without Specter, you need different code for selection vs transformation"

The bidirectionality principle: A path is a lens that focuses on parts of a structure.

Navigator Protocol

abstract type Navigator end

# Core operations - bidirectional by design
function nav_select(nav::Navigator, structure, next_fn)
    # Traverse and collect
end

function nav_transform(nav::Navigator, structure, next_fn)
    # Traverse and modify
end

Primitive Navigators

Navigator	Select Behavior	Transform Behavior
ALL	Each element	Map over all
FIRST	First element	Update first only
LAST	Last element	Update last only
keypath(k)	Value at key	Update value at key
pred(f)	Stay if f(x) true	Transform if f(x) true

Composition: comp_navs (The Key to Performance)

Nathan Marz's critical insight: composition is just allocation + field sets.

Why This Matters

Traditional approaches compile/interpret paths at composition time. Specter does zero work at composition - it just creates an object:

# Specter's key to performance: ONLY allocation + field sets
struct ComposedNav <: Navigator
    navs::Vector{Navigator}  # Just a field - no processing
end

# comp_navs does ONE thing: allocate and set field
comp_navs(navs::Navigator...) = ComposedNav(collect(navs))
# That's it. No compilation. No interpretation. No optimization.
# Just: new ComposedNav() + set navs field

The Magic: Work Happens at Traversal

All the actual work happens when you call select or transform:

# Chain of continuations - CPS (continuation-passing style)
function nav_select(cn::ComposedNav, structure, next_fn)
    function chain_select(navs, struct_val)
        if isempty(navs)
            next_fn(struct_val)  # Base case: call continuation
        else
            # Recursive case: process first nav, chain the rest
            nav_select(first(navs), struct_val, 
                      s -> chain_select(navs[2:end], s))
        end
    end
    chain_select(cn.navs, structure)
end

Why CPS + Lazy Composition = Fast

Traditional:
  compose(a, b, c) → [compile a+b+c] → CompiledPath
  
Specter:
  comp_navs(a, b, c) → ComposedNav{[a, b, c]}  # Just store refs
  select(path, data) → [chain continuations] → results

Benefits:

1. O(1) composition - just allocate, no work
2. Inline caching - same ComposedNav reused at callsite
3. Late binding - dynamic navs resolved at traversal time
4. No intermediate allocations - CPS avoids building result lists

Inline Caching Pattern

# At each callsite, the path is compiled ONCE and cached:
@compiled_select([ALL, pred(iseven)], data)

# Expands to something like:
let cached_nav = nothing
    if cached_nav === nothing
        cached_nav = comp_navs(ALL, pred(iseven))  # First call only
    end
    nav_select(cached_nav, data, identity)  # Reuse forever
end

This is why Specter achieves near-hand-written performance despite the abstraction.

S-expression Navigators

Unique to Julia - navigate typed AST nodes:

# Type definitions
abstract type Sexp end
struct Atom <: Sexp
    value::String
end
struct SList <: Sexp
    children::Vector{Sexp}
end

# Navigators
SEXP_HEAD      # → first(children)
SEXP_TAIL      # → children[2:end]
SEXP_CHILDREN  # → children vector
SEXP_WALK      # Recursive prewalk
sexp_nth(n)    # → children[n]
ATOM_VALUE     # → atom.value

Example: AST Transformation

sexp = parse_sexp("(define (square x) (* x x))")

# Rename function
renamed = transform(
    [sexp_nth(2), sexp_nth(1), ATOM_VALUE],
    _ -> "cube",
    sexp
)
# → (define (cube x) (* x x))

ACSet Navigators

Navigate category-theoretic databases:

# Navigate morphism values
acset_field(:E, :src)

# Filter parts by predicate
acset_where(:E, :src, ==(1))

# All parts of an object
acset_parts(:V)

Example: Graph Transformation

g = @acset Graph begin V=4; E=3; src=[1,2,3]; tgt=[2,3,4] end

# Select: get all source vertices
select([acset_field(:E, :src)], g)  # → [1, 2, 3]

# Transform: shift targets
g2 = transform([acset_field(:E, :tgt)], t -> mod1(t+1, 4), g)

Dynamic Navigators

selected(subpath)

Stay at current position if subpath matches:

# Select values > 5
select([ALL, selected(pred(x -> x > 5))], [1,2,3,4,5,6,7,8,9,10])
# → [6, 7, 8, 9, 10]

if_path(cond, then, else)

Conditional navigation:

if_path(pred(iseven),
        keypath(:even_branch),
        keypath(:odd_branch))

Coercion (Like Specter's coerce-nav)

coerce_nav(x::Navigator) = x
coerce_nav(s::Symbol) = keypath(s)
coerce_nav(f::Function) = pred(f)
coerce_nav(v::Vector) = comp_navs(coerce_nav.(v)...)

API

# High-level interface
select(path, data)                    # Collect matches
select_one(path, data)                # Single match or nothing
transform(path, fn, data)             # Transform matches
setval(path, value, data)             # Set matches to value

Comparison: Clojure vs Julia

Clojure (Specter)	Julia (SpecterACSet)	Notes
(select [ALL even?] data)	select([ALL, pred(iseven)], data)	Same pattern
(transform [ALL even?] f data)	transform([ALL, pred(iseven)], f, data)	Bidirectional
Keywords implicit	keypath(:k) explicit	Type safety
No ACSet support	acset_field, acset_where	Category theory
No typed sexp	Atom/SList discrimination	AST navigation

GF(3) Triads

three-match (-1) ⊗ specter-acset (0) ⊗ gay-mcp (+1) = 0 ✓
lispsyntax-acset (-1) ⊗ specter-acset (0) ⊗ cider-clojure (+1) = 0 ✓

Files

• Implementation: lib/specter_acset.jl
• Babashka comparison: lib/specter_comparison.bb

References

• Specter GitHub
• Nathan Marz: "Rama on Clojure's Terms" (2024)
• Nathan Marz: "Specter: Powerful and Simple Data Structure Manipulation"
• Lens laws (Haskell perspective)