Question
Space leak in recursive accumulation with foldl
Sky Finch
0 reputation · 07 May 2021, 09:42 UTC
125.1K views
Loading…
0 reputation · 07 May 2021, 09:42 UTC
Haskell's default lazy evaluation defers computations by creating thunks. When using a recursive accumulation pattern, such as with foldl, these unevaluated expressions can build up in memory, leading to a space leak before the final result is demanded.
While foldl' and the BangPatterns extension are documented methods to force evaluation to Weak Head Normal Form (WHNF), there is uncertainty regarding the depth of this evaluation. Strictness annotations on data fields ensure the constructor is evaluated, but nested structures within those fields may still retain thunks.
Given GHC's strictness analyzer and varying optimization levels (e.g., -O2), the exact timing of evaluation and the resulting memory footprint can shift.
BangPatterns on a top-level accumulator?undefined) affect the point of failure in a strict fold?34775 reputation · 07 May 2021, 11:51 UTC
No, the GHC strictness analyzer and BangPatterns do not consistently eliminate thunks in nested structures. A bang pattern on a top-level accumulator forces evaluation to Weak Head Normal Form (WHNF). For a simple type like Int, this is sufficient. However, for nested structures (e.g., a tuple or a custom data type), WHNF only evaluates the outermost constructor. The values inside that constructor remain as thunks unless they are specifically marked as strict in the data definition or forced via deepseq.
The point of failure when encountering undefined depends entirely on the evaluation strategy:
haskellfoldl): The error is deferred. The fold builds a massive chain of thunks representing the computation. The program will only crash when the final result is demanded and the evaluator attempts to reduce the thunk chain, hitting the undefined value then.foldl'): The error is immediate. Because foldl' forces the accumulator to WHNF at every step, it will encounter the undefined value as soon as that specific element is processed during the accumulation phase.To resolve space leaks in recursive accumulation, follow these steps in order:
foldl with foldl': This prevents the top-level thunk chain from growing linearly with the input size.data Acc = Acc !Int !Int). This ensures that forcing the Acc constructor also forces the internal values.deepseq for Complex Nesting: If the accumulator contains deeply nested structures that cannot be made strict via data definitions, use Control.DeepSeq to force full evaluation.You can verify the behavior of your fold using GHC profiling. Compile with -prof -fprof-auto and run the executable with +RTS -hc to generate a heap profile. A space leak will appear as a linear growth in THUNK memory over time.
Missing Diagnostic: To provide a more specific recommendation, are you accumulating into a primitive type (like Int) or a composite data structure (like a Map or custom Record)?
Use comments to ask for clarification. Post a solution as an answer.
34,775 reputation · 07 May 2021, 14:38 UTC
To move beyond theoretical strictness and verify if the GHC strictness analyzer is actually eliminating thunks in your specific recursive loop, you can inspect the Core representation. Using the -ddump-simpl flag during compilation allows you to see if the compiler has inserted the necessary strictness checks or if it is still generating thunks for nested fields.
When reviewing the Core, look for the following indicators:
let bindings around your accumulator updates often signals that thunks are being deferred.For runtime confirmation, running the executable with +RTS -s provides a summary of total memory allocation. A linear increase in memory relative to the input size, despite using foldl', is a primary indicator that nested thunks are still accumulating.