The compiler emits eventlog markers at two levels:

• Module-level: "ModuleName start" / "ModuleName end" (from Make.hs)
• Declaration-level: "tc ModuleName kind:name start" / "tc ModuleName kind:name end" (from TypeChecker.hs)

Declaration kinds: val, bind (recursive group), data, datagroup, syn, kind, class, instance, extern, externdata, role.

# 1. Build the compiler (eventlog is enabled by default in GHC >= 9.2)
stack build

# 2. Run with eventlog enabled, single-threaded for clean nesting
cd /path/to/your/purescript/project
purs +RTS -l-agu -N1 -RTS compile $(spago sources)

# 3. Convert the binary eventlog to JSON
eventlog2html --json purs.eventlog

# 4a. Text report (module + declaration breakdown)
node /path/to/purescript/debug/eventlog.js purs.eventlog.json

# 4b. Flamegraph (Chrome trace format — works in speedscope and chrome://tracing)
node /path/to/purescript/debug/eventlog-chrome-trace.js purs.eventlog.json > profile.json
# Open profile.json at https://www.speedscope.app/ or chrome://tracing

Without -N1, modules typecheck in parallel and declaration events from different modules interleave. The speedscope converter handles this, but the flamegraph is cleaner with -N1.

node debug/eventlog.js purs.eventlog.json

Prints per-module timing (sorted ascending), concurrency stats, and a per-declaration breakdown showing the top 50 slowest declarations with module name, declaration kind, wall-clock time, and percentage.

node debug/eventlog-chrome-trace.js purs.eventlog.json > profile.json

Outputs Chrome trace format JSON. Open in:

• https://www.speedscope.app/ (drag and drop)
• chrome://tracing in Chrome (load file)

The flamegraph shows module-level and declaration-level spans nested properly. The text report (top N declarations) is also printed to stderr.

Options:

• --top N — number of declarations in stderr report (default: 50)
• --cap CAP — filter to a specific GHC capability (thread)

The flamegraph has two levels:

1. Module — total time for the module (includes desugaring, codegen, etc.)
2. Declaration — time for typechecking each declaration within the module

This tells you which declarations are expensive. Common patterns:

• Large instance: spans — complex instance resolution, often involving row-list traversals or EncodeJson/DecodeJson generics
• Large bind: or val: spans — complex type inference with many constraints
• Large datagroup: spans — mutually recursive type definitions with many constructors

The traceMarker calls are effectively free when not profiling:

• GHC >= 9.2: eventlog is always linked; traceMarker checks a flag and returns
• Per call: ~tens of nanoseconds (buffer write)
• Total for ~10k declarations: ~1ms against a ~72s build

Install eventlog2html:

cabal install eventlog2html
# or
pip install eventlog2html