Perish or Flourish? A Holistic Evaluation of Large Language Models for Code Generation in Functional Programming

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Hauptverfasser: Lang, Nguyet-Anh H., Lang, Eric, Le-Cong, Thanh, Le, Bach, Huynh, Quyet-Thang
Format: Preprint
Veröffentlicht: 2026
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author Lang, Nguyet-Anh H.
Lang, Eric
Le-Cong, Thanh
Le, Bach
Huynh, Quyet-Thang
author_facet Lang, Nguyet-Anh H.
Lang, Eric
Le-Cong, Thanh
Le, Bach
Huynh, Quyet-Thang
contents Functional programming provides strong foundations for developing reliable and secure software systems, yet its adoption remains not widespread due to the steep learning curve. Recent advances in Large Language Models (LLMs) for code generation present new opportunities to lower these barriers. However, extensive evaluations of LLMs largely focus on imperative programming languages, and their capabilities in functional programming languages (FP) remain underexplored. To address this gap, we introduce FPEval, a holistic evaluation framework built on FPBench, a new benchmark of 721 programming tasks across three difficulty levels on three mainstream FP languages: Haskell, Ocaml and Scala. FPEval provides compehensive evaluation infrastructures with both test validations with comprehensive test suites and static analysis tools to assess both functional correctness and code style and maintainability. Using this framework, we evaluate state-of-the-art LLMs, including GPT-3.5, GPT-4o, and GPT-5, for code generation in functional programming languages and Java as an imperative baseline. Our results demonstrate that LLM performance in functional programming improves substantially with model advancement; however, error rates remain significantly higher in purely functional languages (Haskell and OCaml) than in hybrid (Scala) or imperative (Java) languages. Moreover, LLMs frequently generate non-idiomatic functional code that follows imperative patterns, raising concerns about code style and long-term maintainability. Finally, we show that LLMs can partially self-repair both correctness and quality issues when provided with static analysis feedback and hand-crafted instructions for common types of issues.
format Preprint
id arxiv_https___arxiv_org_abs_2601_02060
institution arXiv
publishDate 2026
record_format arxiv
spellingShingle Perish or Flourish? A Holistic Evaluation of Large Language Models for Code Generation in Functional Programming
Lang, Nguyet-Anh H.
Lang, Eric
Le-Cong, Thanh
Le, Bach
Huynh, Quyet-Thang
Programming Languages
Artificial Intelligence
Software Engineering
Functional programming provides strong foundations for developing reliable and secure software systems, yet its adoption remains not widespread due to the steep learning curve. Recent advances in Large Language Models (LLMs) for code generation present new opportunities to lower these barriers. However, extensive evaluations of LLMs largely focus on imperative programming languages, and their capabilities in functional programming languages (FP) remain underexplored. To address this gap, we introduce FPEval, a holistic evaluation framework built on FPBench, a new benchmark of 721 programming tasks across three difficulty levels on three mainstream FP languages: Haskell, Ocaml and Scala. FPEval provides compehensive evaluation infrastructures with both test validations with comprehensive test suites and static analysis tools to assess both functional correctness and code style and maintainability. Using this framework, we evaluate state-of-the-art LLMs, including GPT-3.5, GPT-4o, and GPT-5, for code generation in functional programming languages and Java as an imperative baseline. Our results demonstrate that LLM performance in functional programming improves substantially with model advancement; however, error rates remain significantly higher in purely functional languages (Haskell and OCaml) than in hybrid (Scala) or imperative (Java) languages. Moreover, LLMs frequently generate non-idiomatic functional code that follows imperative patterns, raising concerns about code style and long-term maintainability. Finally, we show that LLMs can partially self-repair both correctness and quality issues when provided with static analysis feedback and hand-crafted instructions for common types of issues.
title Perish or Flourish? A Holistic Evaluation of Large Language Models for Code Generation in Functional Programming
topic Programming Languages
Artificial Intelligence
Software Engineering
url https://arxiv.org/abs/2601.02060