Here’s an example > length (undefined : 3 : 4 : undefined : []) 4 > length (2 : 3 : 4 : 5 : undefined) <<loop>> The first list contains bottoms as elements, but the “shape” of the list is fully defined. Roughly speaking, every list cell has a clearly defined “pointer” to its … Read more
Unboxed vs Boxed Data To support parametric polymorphism and laziness, by default Haskell data types are represented uniformly as a pointer to a closure on the heap, with a structure like this: (source: haskell.org) These are “boxed” values. An unboxed object is represented by the value itself directly, without any indirection or closure. Int is … Read more
Unless you’re storing a large computation in the Int and Float fields, significant overhead can build up from lots of trivial computations building up in thunks. For instance, if you repeatedly add 1 to a lazy Float field in a data type, it will use up more and more memory until you actually force the … Read more
A good place to start is by understanding this paper: A Natural Semantics for Lazy Evalution (Launchbury). That will tell you when expressions are evaluated for a small language similar to GHC’s Core. Then the remaining question is how to map full Haskell to Core, and most of that translation is given by the Haskell … Read more
I’ll try to give an explanation in simple terms. As others have pointed out, head normal form does not apply to Haskell, so I will not consider it here. Normal form An expression in normal form is fully evaluated, and no sub-expression could be evaluated any further (i.e. it contains no un-evaluated thunks). These expressions … Read more