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Pine v6's type system, in practice

PineScript v6 is more strongly typed than its reputation suggests. Walking through the type rules, the inference, the gotchas — and what shipping a transpiler taught us about which rules actually matter at runtime.

10 min read#pine-script#types#language

Pine Script has a reputation for being loosely typed. The reputation is partly deserved and mostly wrong. It earned the label because int widens to float silently, na is accepted almost everywhere, and the language rarely demands an explicit annotation. But underneath that permissive surface is a type system with real teeth — and once you need to transpile it to a statically typed language, every tooth matters.

This post walks through what Pine v6's types actually are, where the inference layer hides the complexity, and the three gotchas we hit most often while building the C++ codegen.

Reputation vs. reality

A new Pine programmer writes:

x = 5
y = close + x

No annotations. No complaints from the compiler. The programmer concludes "Pine has no types." But Pine inferred exactly what those variables are: x is a simple int (a compile-time constant integer), and y is a series float (a per-bar floating-point value). The inference is doing real work; it's just invisible.

The distinction between simple int and series float isn't cosmetic. They have different runtime behaviors, different operator compatibility rules, and different representations in a transpiled output. The language is typed; it just doesn't force you to write the types down.

The type kinds

Pine v6 has four layers to its type system. Most users encounter only the first.

Primitives

The primitive types are int, float, bool, string, and color. There's also na, which deserves its own section below. These are the leaf types — the ones that hold scalar values.

int and float participate in implicit widening: when you mix them in an expression, the int widens to float. This is the behavior that earns Pine the "loosely typed" label, but it's a deliberate and consistent rule, not a special case. Every strongly typed language does the same thing for numeric types.

Composites

Pine v6 supports three composite types: array<T>, matrix<T>, and map<K, V>. All three are parameterized by their element type, which must be a Pine type (including UDTs). You can have array<float>, array<int>, even array<MyUDT>.

Composite types are mutable objects. They're reference types in the sense that passing an array<float> to a function gives the function a handle to the same underlying storage. If the function modifies the array, the caller sees the change.

User-defined types (UDTs)

Pine v6 added the type keyword, which lets you define named record types with fields and optional methods:

type Order
    float price
    int   qty
    bool  filled = false
 
method fill(Order this, float fillPrice) =>
    this.price  := fillPrice
    this.filled := true

UDTs are Pine's closest analog to structs. They can have default values for fields (as shown with filled = false). Methods are called with dot notation and receive the UDT instance as this. Fields are mutable via := assignment.

Form qualifiers: series vs. simple

Every value in Pine — not just primitives, but composites and UDTs too — carries a form qualifier that describes its relationship to the bar timeline.

The form qualifiers form a hierarchy: const ⊂ simple ⊂ series. An expression that mixes forms is promoted to the highest (most general) form. simple int + series float is series float.

na semantics

na is not a universal null. It's a typed value — na<int> and na<float> are distinct, and the type is inferred from context. In an expression like:

x = na

Pine infers x as series float by default (the most commonly useful type for an uninitialised series). In a context where the type is constrained — say, you later write x := 5 — the inference locks in int.

Arithmetic on na propagates: na + 1 is na. This is intentional and consistent — if a value is unknown, any expression depending on it is also unknown. The escape hatch is nz(x, 0), which replaces na with a default value.

Comparison operators follow the same rule: na == na is na, not true. This is the gotcha that bites most often. If you want to test whether a value is missing, use na(x) (the built-in predicate), not x == na.

In our C++ codegen, na maps to std::optional<T>. Arithmetic on std::nullopt produces std::nullopt. The na(x) predicate becomes !x.has_value(). The semantics map cleanly; the verbosity is higher in C++.

Type inference in detail

Pine infers types from the right-hand side of assignments. The rules are straightforward once you know the form qualifier hierarchy:

a = 5           // const int
b = 5.0         // const float
c = close       // series float  (close is always series float)
d = close + 1   // series float  (int widens to float; series promotes)
e = bar_index   // series int    (bar_index is series int, always)

The interesting case is inference across branches:

x = condition ? close : 0

condition might be series bool. close is series float. 0 is const int. Pine widens 0 to series float and the result is series float. The compiler always picks the most general form and the most encompassing type.

Function arguments constrain inference too. If a built-in expects simple int and you pass bar_index (which is series int), you get a compile error. This is where the type system's strictness surfaces: you can't pass a bar-varying value to a parameter that requires a fixed-at-startup value.

How the C++ codegen represents these types

Translating Pine's type system to C++ requires mapping each Pine form and type to a C++ representation that has the right runtime behavior.

Primitives map to double, int64_t, bool, and std::string. float in Pine is always 64-bit; we use double throughout.

Series are represented as templated lazy-evaluated history buffers. The abstraction exposes:

template <typename T>
class Series {
public:
    T current() const;
    T at(int bars_ago) const;  // implements the [] operator
    void advance(T next_value);
};

When Pine code writes close[2], the codegen emits close.at(2). When a bar completes, the engine calls advance() on every series to shift the history window.

na maps to std::optional<T>. Arithmetic operators on Series<std::optional<T>> propagate nullopt exactly as Pine propagates na. The nz() built-in becomes x.value_or(default_value).

UDTs become plain structs. Fields become struct members. Methods become member functions. The Pine field-assignment operator := becomes a plain C++ assignment; there are no setters.

Arrays and matrices become std::vector<T> and a thin 2D wrapper around std::vector<T>, respectively.

The three gotchas we hit most

1. Mixing series and simple promotes to series — even when you expected a constant

myLength = 14
ma = ta.sma(close, myLength)

If myLength is declared in the global scope and never changes, you might expect the codegen to treat it as a compile-time constant. Pine infers it as simple int, which is not const — the value is fixed at runtime startup, not at compile time. The consequence is that ta.sma(close, myLength) is series float, not something the compiler can fold.

This matters in our codegen because we wanted to const-fold certain window sizes for performance. Pine's type system correctly refuses: simple is not const, and a variable declared without the const keyword could in principle be set via input.int() — which makes it startup-determined, not compile-determined.

2. UDT field assignment doesn't trigger methods — and that's intentional

type Box
    float value
 
method doubled(Box this) =>
    this.value * 2
 
b = Box.new(value = 10.0)
b.value := 20.0   // direct field write, no notification

In an object-oriented language, you might expect b.value := 20.0 to go through a setter that could call doubled() or trigger any other behavior. In Pine, it doesn't. Fields are public and directly mutable. Methods are just functions scoped to the type. If you're building a state machine in a UDT and you want validation on write, you must explicitly call a method that does the validation — field writes bypass everything.

We document this internally because it's the main place where a Pine program can look like it has encapsulation but doesn't. The C++ codegen reflects this accurately: field writes are plain struct member assignments.

3. request.security defaults to causal — but the lookahead default has changed across versions

htf_close = request.security("BTCUSDT", "D", close)

This call is type-correct and compiles without warnings. What it returns is the daily close as of the current bar's time. If the current bar is a 15-minute bar mid-day, htf_close carries the previous day's close until the new daily bar closes — that's the causal interpretation.

The historical gotcha: in Pine v4, request.security defaulted to lookahead=barmerge.lookahead_on, which allowed peeking at the future daily bar before it closed. This inflated backtest results for strategies that used HTF data. Pine v5 changed the default to lookahead_off (causal), which is the correct behavior. Pine v6 preserves the v5 default.

The type system doesn't warn you about this because both modes are type-correct. The PineForge codegen always applies lookahead_off semantics by default, matching the Pine v6 reference behavior. If you're porting a strategy from v4 and your backtest results look surprisingly different on PineForge — this is likely why.

Why transpiling Pine made us more rigorous

Every ambiguity in Pine's type rules has to become a decision in the C++ codegen. When Pine's reference documentation says "this is implementation-defined" or when two behaviors are both technically consistent with the spec, we can't pick one and hope. We have to test against TradingView's actual output, figure out which branch TradingView took, and implement that branch.

This process has made us read the Pine v6 language reference more carefully than any user would bother to. It's also generated most of our parity test cases: when we find a corner case in request.security or strategy.exit where our codegen disagrees with TradingView, we write a minimal reproducer, add it to the validation corpus, and fix the codegen until both outputs match.

The type system is where corner cases live. The form qualifiers — series vs. simple vs. const — look like a minor implementation detail, but they determine which operations are legal, which values can be passed to which functions, and which expressions can be evaluated at compile time rather than bar by bar. Getting them right is the difference between a transpiler and a good one.

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