Programming Tutorials

  In Haskell, the elegance of lazy evaluation comes with a specific operational cost: the space leak. You write a mathematically pure function to fold over a large dataset, it compiles perfectly, but at runtime, it either consumes all available RAM or crashes with   Stack space overflow . This failure usually stems from  thunk buildup —the accumulation of unevaluated expressions in memory. This post analyzes the mechanics of this failure, demonstrates how to profile it using GHC's tooling, and provides a robust solution using strictness annotations. The Root Cause: Lazy Evaluation and Thunks By default, Haskell is lazy. When you bind a variable or pass an argument, the runtime does not evaluate the expression immediately. Instead, it creates a  thunk —a pointer to the code required to calculate the value and its execution environment. In a recursive operation like  foldl , this behavior is dangerous. Consider a naive average calculation: -- BAD: This builds a ma...

  The most insidious failure mode in Haskell production systems is the slow-burning memory leak. Your application runs perfectly for days, but the Resident Set Size (RSS) creeps upward until the OOM killer terminates the process. Standard heap profiling ( -hT ) often changes the runtime characteristics enough to hide the bug (the "Heisenbug" effect) or requires restarting the process, destroying the state you need to inspect. The modern solution is  ghc-debug . This toolset allows you to connect to a running Haskell process, inspect the heap graph programmatically, and identify thunk buildup without stopping the world for extended periods or recompiling with heavy instrumentation. The Root Cause: Thunks and WHNF Haskell’s memory leaks are rarely "leaks" in the C/C++ sense (unfreed memory). They are almost always  unwanted retention . Because Haskell is lazy, an expression like  acc + 1  is not evaluated immediately. It allocates a "thunk" (a closure repres...