Where can I get some paper/doc/whatever which describes how a Haskell compiler actually works? I read quite a few of the docs of GHC, but stopped after getting a headache. So, something which doesn't require a PhD to understand it and isn't written in the You're-supposed-to-be-already-familiar-with-it style would be preferable. It's not a problem if it's really long and takes some time to understand it though.
PS: Most interesting would be something about GHC, but anything is ok.
Haskell is statically typed. When you compile your program, the compiler knows which piece of code is a number, which is a string and so on. That means that a lot of possible errors are caught at compile time. If you try to add together a number and a string, the compiler will whine at you.
The compiler (written in Haskell), translates Haskell to C, assembly, LLVM bitcode and other formats. The strategy it uses is described best here: Implementing lazy functional languages on stock hardware:the Spineless Tagless G-machine.
Haskell has an open, published specification, and multiple implementations exist. Its main implementation, the Glasgow Haskell Compiler (GHC), is both an interpreter and native-code compiler that runs on most platforms.
Start Haskell If you have installed the Haskell Platform, open a terminal and type ghci (the name of the executable of the GHC interpreter) at the command prompt. Alternatively, if you are on Windows, you may choose WinGHCi in the Start menu. And you are presented with a prompt.
You can get an answer from the horse's mouth! Simon Peyton Jones (GHC wizard) wrote a book explaining how to implement functional programming languages. It's available for free online since it's now out of print: http://research.microsoft.com/en-us/um/people/simonpj/papers/pj-lester-book/
Of course, GHC has moved on since the book was written, but it's still very relevant.
Are you looking for details especially about compiling lazy-evaluation? There is Simon Peyton-Jones's book mentioned by Max Bolingbroke, also the book detailing Clean's implementation is online:
http://wiki.clean.cs.ru.nl/Functional_Programming_and_Parallel_Graph_Rewriting
If you have a university affiliation and want something smaller you could try to get these books (Henderson & Diller are certainly out of print):
Antoni Diller "Compiling Function Languages" ISBN 0 471 92027 4
Peter Henderson "Functional Programming Application and Implementation" ISBN 0-13-331579-7
AJT Davie "An Introduction to Functional Programming Systems using Haskell" ISBN 0 521 27724 8
Diller has a full compiler for a lazy language (implemented in Pascal) via combinator reduction. This was the implementation technique invented by David Turner for SASL. Henderson has many parts of a compiler for LISPkit a miniature, lazy variant of Lisp. Davie details quite a bit of the machinery for compiling a lazy language, for instance there's a description of the STG thats much shorter than Simon Peyton-Jones's book (the STG is the abstract machine SPJ used for Haskell).
The Clean developers have quite a bit of info on implementing SAPL (a Simple Applicative Language) if you look through their publications list:
https://clean.cs.ru.nl/Publications
Finally there are quite a number of papers documenting aspects of the Utrecht Haskell Compiler UHC (and EHC). I think most of the information is how the compiler is organized (with attribute grammars and "Shuffle") and how the type systems (there are various levels of type system in EHC) are implemented, rather than how the back-end 'compilation' works.
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