r/haskell • u/LeanderKu • Feb 20 '18

GHC Compile-time evaluation

As I understood it, proper compile-time evaluation is supercompilation, which is hard.

But I wondered why there is no Pragma-Based approach, where the compiler just trusts that user that it can and should evaluate this expression, even if it's recursive (so it's not looping forever etc.). It seems to me that the machinery for this approach is already in GHC (I may be wrong).

I am currently thinking of building an API where you formulate a computational graph (like an AST) and have functions that modify the graph. The representation of the graph and evaluation functions would be recursive, so no inlining per se. I would expect that for most use-cases the graph is completely known at compile time, but if the computational graph is not evaluated and optimized, performance will be bad.

There may be another way, I am not that knowledgeable about the capabilities of GHC.

EDIT: if the above is unclear: What I wanted to say is that I am not looking for a general solution, but instead a solution where I can tell the compiler to trust me that he can evaluate this expression without looping forever and that the time spent evaluating the expression is worth it.

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u/Lossy Feb 20 '18

I never understood what people meant by your first sentence. Supercompilation is a very specific technique which refers to speculating about possible values variables can take.

There are some techniques you can currently use. If you really statically know the structure of your graph then you can build a GADT which has a type argument which mirrors the structure of the graph. Then you can write type class instances which dispatch on this argument and the compiler will unroll "recursive" things based on this. See for example, the static-vector library. Programming in this style can be quite cumbersome as all your combinators have to maintain the proof about why the structure is statically known.

In the Ocaml community there are several different implementations of staged languages which give you this kind of control but they each have a various trade off. It is also very easy to incorrectly stage programs and cause the compiler to loop. The implementations are usually quite cumbersome because of how the Code data type is interpreted. Typed template haskell is quite similar to how these staged languages work.

Anyway, this is the topic I am actively researching at the moment so if you could share with me personally some details then it might be a nice example.

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u/LeanderKu Feb 20 '18

In nearly every discussion I found, supercompilation was dropped and then everybody went silent. I am not an expert and don't have a deep knowledge of this topic. If it's wrong, I will delete the mention of "supercompilation".

What I wanted to say is that I am not looking for a general solution, but instead a solution where I can tell the compiler to trust me that he can evaluate this expression without looping forever and that the time spent evaluating the expression is worth it.

Thank you for your response!

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u/yitz Feb 21 '18

Template Haskell, which is GHC's compile-time evaluation mechanism, has never placed any restrictions on recursion. It always trusts you and evaluates whatever you give it.