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feat(interp): an evaluator with its own stack on the heap - #443
The interpreter was a recursive tree walker. Each call of the program used native stack, and the process aborted on deep recursion:
Native stack for each call
Greatest depth (stack of 8 MB)
Debug build
51 KB
161
Release build
9 KB
907
Wasm (for reference)
16250, then the trap "call stack exhausted"
eval is now a loop with a stack of frames on the heap (owner decision 2026-11-10). A call uses no native stack.
This is pull request 1 of 2. Pull request 2 changes the values (see "Not in this pull request").
Changes
All in src/ephapax-interp/src/lib.rs.
Code form.lower turns an Expr into a Node tree with shared parts (Arc), 1 time. A frame can then hold the part that runs next, with no lifetime and no unsafe.
The loop.run holds a state (code to evaluate, or a value) and a Vec<Frame>. enter starts a node; resume gives a value to the top frame. The frames are: Unary, Left, Right, Let, If, Case, Match, Guard, Seq, Unbind, Env, Region.
The limit. More than 1000000 frames give the new error RuntimeError::StackOverflow, with the text "call stack exhausted" (the text of the wasm trap).
Functions. A top-level function is lowered when its module is loaded. The old code copied the body of the function at each call.
Value::Closure. Its body is Code (an opaque shared pointer), not an Expr. No other crate reads that field.
The behaviour of each form is the same: the same order of evaluation, the same errors, and the same state of the environment after an error (the value of a constant gives the environment back; the other forms leave it as it is).
Measurements
Release build, fn sum(n: I32): I32 = if n == 0 then 0 else n + sum(n - 1):
Memory is about 1.3 KB for each call. Most of it is the environment of the call, which holds values of 168 bytes. Pull request 2 makes the values smaller.
Not in this pull request
A Value still copies, prints and frees its parts with native recursion. I measured the size at which a list value overflows the native stack: copy at 1000 (debug) to 6700 (release) elements, print at 800 to 18000, free at 10000 to 64000. So a program that builds a long list can still abort, as it did before at a call depth of 161. Pull request 2 gives the values shared parts, so that a copy is 1 step and print and free use a loop.
Tail calls are not optimised: each call adds 1 frame.
Test plan
3 new unit tests in ephapax-interp: 100000 calls deep give a value on the 2 MB stack of a test thread; calls with no end give StackOverflow; an error in the value of a constant keeps the environment of its user.
New CLI test the_interpreter_runs_deep_calls_and_names_its_limit (run_wasm.rs): depth 100000 through the frames of a match, a let and a call; calls with no end give the error text and exit code 1.
cargo test --workspace: 647 pass, 0 fail. cargo fmt --check and cargo clippy --workspace --all-targets -- -D warnings are clean.
Probe battery: 51 SAME, 0 CHANGED, 7 SKIP (no wasmtime CLI in the session).
run --interp on each of the 149 .eph files of the repository gives the same output and exit code with the old and with the new evaluator.
The interpreter was a recursive tree walker: each call of the program
used native stack (51 KB in a debug build, 9 KB in a release build), and
the process aborted at a call depth of 161 (debug) or 907 (release) on a
stack of 8 MB. Wasm reaches 16250 and gives a named trap.
`eval` is now a loop with a stack of frames on the heap (owner decision
2026-11-10). An expression is lowered 1 time to code with shared parts
(`Node`), so a frame can hold the part that runs next. A call uses no
native stack:
- 100000 calls deep give a value (0.14 s in a release build).
- Calls with no end give the runtime error "call stack exhausted" at
1000000 frames, with exit code 1. The process does not abort.
The behaviour of each form is the same, with the same order of
evaluation and the same errors. An error leaves the environment as the
recursive evaluator left it. A top-level function is lowered when its
module is loaded, not copied at each call: `fib(27)` runs in 0.45 s, was
0.61 s.
`Value::Closure` holds its body as `Code` (an opaque shared pointer),
not as an `Expr`. No other crate reads that field.
Not in this change: a `Value` still copies, prints and frees its parts
with native recursion, so a program that builds a list of more than
about 1000 elements can still abort. That is the next pull request.
Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0136P7ZmKrYLDC4WHi2yt11u
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src/ephapax-cli/tests/run_wasm.rs
src/ephapax-interp/src/lib.rs
Summary
Summary by CodeRabbit
Bug Fixes
The interpreter now handles programs with at least 100,000 recursive calls, including calls through match and let.
Recursion that exceeds the interpreter’s limit now exits with a clear runtime error instead of exhausting the system call stack.
The interpreter now lowers expressions to shared code and evaluates them with heap-backed states and frames instead of recursive expression calls. It reports frame exhaustion as StackOverflow. Tests cover deep recursion, non-terminating recursion, and constant-evaluation environment restoration.
Changes: Interpreter evaluation
Layer / File(s)
Summary
Lowered code and function representation src/ephapax-interp/src/lib.rs
The interpreter adds shared code nodes and lowers expressions into them. Function definitions and closure bodies now store lowered code.
Iterative evaluation and scoped expressions src/ephapax-interp/src/lib.rs
The evaluator uses states and frames for expression evaluation, scope handling, branches, matches, and regions. It returns StackOverflow when the frame count exceeds 1,000,000.
Runtime operations and function calls src/ephapax-interp/src/lib.rs
Runtime operations consume evaluated values. Function calls schedule lowered bodies, and constant evaluation uses frames to restore the caller environment.
Recursion and environment validation src/ephapax-interp/src/lib.rs, src/ephapax-cli/tests/run_wasm.rs, TEST-NEEDS.adoc
Tests cover deep calls, frame exhaustion, and environment preservation after a constant-evaluation error. The CLI test checks deep and non-terminating recursion. The documented all-target test total changes to 644.
sequenceDiagram
participant Interpreter
participant lower
participant run
participant Frames
Interpreter->>lower: Convert expression to Code
lower-->>Interpreter: Return lowered Code
Interpreter->>run: Evaluate Code
run->>Frames: Push and resume evaluator frames
Frames-->>run: Return value or error
run-->>Interpreter: Return evaluation result
Loading
Possibly related PRs
hyperpolymath/ephapax#441: Adds top-level constant evaluation, which this change now handles through evaluator frames.
hyperpolymath/ephapax#431: Adds guarded match behaviour, which this change evaluates through heap-backed frames.
hyperpolymath/ephapax#429: Adds specialised function declarations that the updated interpreter loads and executes.
Merge Risk: Low · previous review c80c3
Deep recursion now reports a named error instead of crashing the process. A runaway program can use a substantial amount of memory before that error, which could matter on memory-limited hosts. This is worth measuring but should not block merge.
Pre-merge checks: 5
Check name
Status
Explanation
Title check
The title clearly identifies the main change: the interpreter now uses a heap-backed evaluator stack.
Description check
The description explains the evaluator changes, their behaviour, limitations, measurements and test results. It is related to the changeset.
Docstring Coverage
Docstring coverage is 82.35% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentati…
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
Full details: Docstring Coverage
Explanation
Docstring coverage is 82.35% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 17 functions across 2 files. (1 skipped: 1 unsupported.)
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Actionable comments posted: 1
Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix all issues
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Inline comments:
Review comments at @src/ephapax-interp/src/lib.rs:
- Line 374: Reduce peak memory in the recursive no-end path so it returns the
named StackOverflow error before a memory-limited host terminates the process.
Review the retained Frame::Right and Frame::Env payloads and measure peak RSS in
calls_with_no_end_are_a_named_error; keep boxing only if it reduces total memory
use. If memory remains excessive, lower MAX_FRAMES while preserving support for
the 100,000-call tests.
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TEST-NEEDS.adoc
src/ephapax-cli/tests/run_wasm.rs
src/ephapax-interp/src/lib.rs
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After an error, the environment stayed as it was at the error: the
names of the call that failed stayed bound, and a region stayed active.
The recursive evaluator did the same, but calls with no end are now an
error that a caller can catch (the REPL), so the state is visible.
An error now ends each frame, the top one first: the names, the
environment and the regions are as they were before the evaluation.
`Interpreter::call` also restores its environment after an error.
Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0136P7ZmKrYLDC4WHi2yt11u
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A call in tail position found a frame on top that gives the environment
back already. The call now adds no frame of its own, so a loop written
as a tail call has no depth limit: 1200000 calls run in 15 MB.
Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0136P7ZmKrYLDC4WHi2yt11u
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## Summary
Pull request 2 of 2 for the interpreter. Pull request 1,
[#443](#443)
(the evaluator with its own stack), is merged, and the base of this one
is `main`.
A `Value` held its parts in boxes, and it copied, printed and freed them
with native recursion. A list is a chain of 2 parts for each element, so
a long list overflowed the native stack:
| Operation | Elements at the overflow (debug to release, stack of 2 MB
to 8 MB) |
|---|---|
| Copy | 1000 to 6700 |
| Print | 800 to 18000 |
| Free | 10000 to 64000 |
Each read of a variable copied the value. So a function on a list of n
elements needed time and memory of the order n * n.
## Changes
All in `src/ephapax-interp/src/lib.rs`.
- The parts of a value are shared pointers (`Arc`): `Pair`, `Left`,
`Right`, `Borrow`, `Tuple`, `List`, and `Closure(Arc<Closure>)`. A copy
is 1 step.
- `Drop for Value` frees the parts with a loop.
- `Display` prints with a loop. `Debug` is the printed form.
- `is_linear` uses a loop.
- `to_type` gives the type to 64 levels of parts, and the placeholder
`()` below that. The REPL uses it to show a type.
- New constructors: `Value::pair`, `Value::left`, `Value::right`,
`Value::borrow`.
No crate outside the interpreter builds or takes apart a value with
parts.
## Measurements
Release build:
| Program | With #443 only | With this change |
|---|---|---|
| List of 2000 elements (build, then length) | 2.95 s, 3 GB | 0.01 s, 12
MB |
| List of 100000 elements | abort | value, 0.33 s, 179 MB |
| `sum(100000)` | 138 MB | 71 MB |
| Calls with no end | error after 0.53 s, 669 MB | error after 0.37 s,
333 MB |
| `fib(27)` | 0.49 s | 0.41 s |
After this change, no step of the interpreter uses native recursion on
the depth of the calls or on the size of a value. The recursion that
remains is on the nesting of the source text (`lower`, the patterns of a
`match`), as in the parser and in the type checker.
## Test plan
- New unit test `a_long_list_value_uses_no_native_stack`: a list value
of 1000000 elements is copied, printed, typed and freed on the 2 MB
stack of a test thread.
- New unit test `a_value_prints_each_of_its_parts`: the printed form of
each kind of value.
- New CLI test `the_interpreter_runs_a_long_list`
(`recursive_types.rs`): a program builds a list of 100000 elements and
gets its length.
- `cargo test --workspace` after the merge of `main`: 650 pass, 0 fail.
`cargo fmt --check` and `cargo clippy --workspace --all-targets -- -D
warnings` are clean.
- Probe battery: 51 SAME, 0 CHANGED, 7 SKIP (no wasmtime CLI in the
session).
- `run --interp` on each of the 149 `.eph` files of the repository gives
the same output and exit code as before the two pull requests.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
https://claude.ai/code/session_0136P7ZmKrYLDC4WHi2yt11u
---------
Co-authored-by: Claude <noreply@anthropic.com>
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Summary
The interpreter was a recursive tree walker. Each call of the program used native stack, and the process aborted on deep recursion:
evalis now a loop with a stack of frames on the heap (owner decision 2026-11-10). A call uses no native stack.This is pull request 1 of 2. Pull request 2 changes the values (see "Not in this pull request").
Changes
All in
src/ephapax-interp/src/lib.rs.lowerturns anExprinto aNodetree with shared parts (Arc), 1 time. A frame can then hold the part that runs next, with no lifetime and nounsafe.runholds a state (code to evaluate, or a value) and aVec<Frame>.enterstarts a node;resumegives a value to the top frame. The frames are:Unary,Left,Right,Let,If,Case,Match,Guard,Seq,Unbind,Env,Region.RuntimeError::StackOverflow, with the text "call stack exhausted" (the text of the wasm trap).Value::Closure. Its body isCode(an opaque shared pointer), not anExpr. No other crate reads that field.The behaviour of each form is the same: the same order of evaluation, the same errors, and the same state of the environment after an error (the value of a constant gives the environment back; the other forms leave it as it is).
Measurements
Release build,
fn sum(n: I32): I32 = if n == 0 then 0 else n + sum(n - 1):sum(100000)sum(300000)f(n) = 1 + f(n)(no end)fib(27)Memory is about 1.3 KB for each call. Most of it is the environment of the call, which holds values of 168 bytes. Pull request 2 makes the values smaller.
Not in this pull request
A
Valuestill copies, prints and frees its parts with native recursion. I measured the size at which a list value overflows the native stack: copy at 1000 (debug) to 6700 (release) elements, print at 800 to 18000, free at 10000 to 64000. So a program that builds a long list can still abort, as it did before at a call depth of 161. Pull request 2 gives the values shared parts, so that a copy is 1 step and print and free use a loop.Tail calls are not optimised: each call adds 1 frame.
Test plan
ephapax-interp: 100000 calls deep give a value on the 2 MB stack of a test thread; calls with no end giveStackOverflow; an error in the value of a constant keeps the environment of its user.the_interpreter_runs_deep_calls_and_names_its_limit(run_wasm.rs): depth 100000 through the frames of amatch, aletand a call; calls with no end give the error text and exit code 1.cargo test --workspace: 647 pass, 0 fail.cargo fmt --checkandcargo clippy --workspace --all-targets -- -D warningsare clean.run --interpon each of the 149.ephfiles of the repository gives the same output and exit code with the old and with the new evaluator.Generated by Claude Code