New primitives requiring literal syntax

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#1

From the 9.12.1-alpha2 release notes:

New vector shuffle instructions, such as shuffleFloatX4# :: FloatX4# -> FloatX4# -> (# Int#, Int#, Int#, Int# #) -> FloatX4# . These instructions take two input vectors and a collection of indices (which must be compile-time literal integers)

I thought it is interesting that they are required to be literals. I suppose literals are required because they are guaranteed to be available at compile-time. However, there are other approaches to provide compile time computation:

  • TemplateSyntax, which I imagine the GHC team might want to avoid, given that it has been so hard to get it working with JavaScript and cross compilation?
  • Singletons, which I imagine can’t be used in boot libraries, as it is not considered elegant enough? With singletons, the function itself would be indexed by the unboxed tuple. I haven’t used singletons, so I am not sure whether this would work?

It’s interesting that GHC is improving support for type-level programming, but at the same time, ‘magic’ functions are able to verify whether its inputs are arguments. I use the term ‘magic’ because I imagine that only hardwired functions are able to do something like this. Is that true?

#2

Primops are greatly restricted and can use only few, also primitive types. So anything like singletons is out of question at this level.

A higher-level library for vectorised operations could indeed use TH / singletons in its interface.

#3

They have to be literals by the time they reach the code generator because they are lowered to X86’s SHUFPS which requires an immediate argument.

But as @Bodigrim writes, it’s possible to add higher-level safer interfaces on top of them.

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