ghc

GHC has a lot of optimizations that it can perform, but I don't know what they all are, nor how likely they are to be performed and under what circumstances. My question is: what transformations can I expect it to apply every time, or nearly so? If I look at a piece of code that's going to be executed (evaluated) frequently and my first thought is "hmm, maybe I should optimize that", in which cases should my second thought be, "don't even think about it, GHC got this"? I was reading the paper Stream Fusion: From Lists to Streams to Nothing at All , and the technique they used of rewriting list

I have a code that uses the fromRight function defined circa GHC 8.2. But I need to downgrade to GHC 8.0.2, which gives an error about Variable not in scope: for fromRight I was wondering if it possible and how to add the missing definition fromRight :: b -> Either a b -> b fromRight _ (Right b) = b fromRight b _ = b so that it is only used when I use an GHC version than 8.2.1? You can always write import Prelude hiding (fromRight) which is valid even if fromRight does not exist in Prelude. Therefore, if you want to write a module which is compatible with both old and new versions of Prelude,

I looked at the module of GHC.Prim and found that it seems that all datas in GHC.Prim are defined as data Float# without something like =A|B , and all functions in GHC.Prim is defined as gtFloat# = let x = x in x . My question is whether these definations make sense and what they mean. I checked the header of GHC.Prim like below {- This is a generated file (generated by genprimopcode). It is not code to actually be used. Its only purpose is to be consumed by haddock. -} I guess it may have some relations with the questions and who could please explain that to me. It's magic :) These are the

I'm relatively new to Haskell, and I'm trying to understand one of the definitions of HList . data instance HList '[] = HNil newtype instance HList (x ': xs) = HCons1 (x, HList xs) pattern HCons x xs = HCons1 (x, xs) I have a couple specific questions: What is the '[] and (x ': xs) syntax I'm seeing? It almost looks like it's pattern matching on variadic type parameters, but I have never seen this syntax before, nor am I familiar with variadic type parameters in Haskell. I would guess this is part of GHC's Type Families , but I don't see anything about this on the linked page, and it's rather