wiki:TemplateHaskell/Reify

Making reify useful for typed foreign language bindings generators

ticket:11832

There are a few projects using quasiquotes to interface with other languages. At least C (language-c-inline, inline-c), R (inline-r), and there are ongoing efforts for Java, Javascript and other languages.

A common pattern in these libraries is to collect the content of all quasiquotes that appear in a module, generate a foreign source file based on them, and then link the object code resulting from compiling it.

When the foreign language is itself statically typed, we need to make sure we generate code with proper type annotations, including for any antiquotation variables. In older versions of GHC, we could derive the type annotations from the inferred Haskell types, but this is not possible since 7.8 because the types of any variable in the current "declaration group" are not made available (wiki:​TemplateHaskell/BlogPostChanges). So libraries like inline-c ask for some extra verbosity from the user:

mycossin1 :: Double -> IO Double
mycossin1 somex = [cexp| double { cos($(double somex)) * sin($(double
somex)) } |]

The type annotations inside the quasiquote are redundant with the explicitly provided type signature. C types are typically short enough, but other languages (e.g. C++, Java), have really long fully qualified type names, so the extra annotations are a cost.

The are good reasons why types aren't available from within a declaration group. But by the time type checking of the whole module is finished, types are fixed once and for all. So the question is:

Could we make it possible to query the types of local variables at the end of type-checking?

Template Haskell already provides addModFinalizer. You feed it a Q action, which will run once the whole module is type checked. If at that point we could ask, "what's the type of somex?", then we could let the user write

mycossin2 :: Double -> IO Double
mycossin2 somex = [cexp| cos($somex) * sin($somex) |]

and yet still generate a C file capturing the content of above quasiquote with the right types:

double module_foo_qq1(double v1)
{
    return (cos(v1) * sin(v1));
}

since we know that somex :: Double and that the Haskell type Double corresponds to C's double primitive type.

Bound variables have a unique name associated to them, which we can get hold of in Template Haskell using the 'var syntax, but

f x = $(let name = 'x in addModFinalizer (reify name >>= \info ->
runIO (print info)) >> [| x |])

results in a compiler error,

    The exact Name `x' is not in scope
      Probable cause: you used a unique Template Haskell name (NameU),
      perhaps via newName, but did not bind it
      If that's it, then -ddump-splices might be useful

because by the time the TH finalizer runs, we're no longer in the scope of x.

Can we do anything to improve these use cases of reify?

Proposals

  1. Have addModFinalizer capture the local typing environment at the call site, and run the finalizer on it when type checking completes. Or,
  2. Have a new function addGroupFinalizer like addModFinalizer, which should also capture the local typing environment, and that will run the finalizer on it when typechecking of the current declaration group completes. Or,
  3. Allow reify to observe out-of-scope local variables provided that their names are unique within the module. Or,
  4. Introduce typed quasiquotes, since a quasiquote is syntactic sugar for a splice and typed splices run in the type checker (hence reify has access to the type environment and can therefore return the type of locally defined variables).

(1) and (2) are easy to implement for typed splices, which run in the typechecker. Unfortunately, untyped splices run in the renamer, the local typing environment is not yet populated, and thus it cannot be captured. If we implement addGroupFinalizer in typed splices only, then we still need (4), because currently quasiquotes are syntactic sugar for untyped splices, not typed ones.

The problem with (4) is that running splices in an incompletely determined typing environment is a bad idea: as TemplateHaskell/BlogPostChanges argues, the result of reify is unstable in the face of changes to the type checker. If (4) was restricted somehow, e.g. to only allow reification of variables outside of the current declaration group, then it would make typed quasiquotes useless for the purposes of this particular use case.

So that leaves us with the following options:

  1. Somehow provide addModFinalizer/addGroupFinalizer with the local type environment of the call site.
  2. Implement option (3).

Design proposal for implementing addGroupFinalizer in untyped splices

A proposal:

  • When the renamer runs addGroupFinalizer it annotates the AST with a mutable reference at the location of the splice.
    r <- newIORef (Nothing :: Maybe LocalTypeEnv)
    
    Perhaps the annotation could be implemented by extending the Tickish datatype with a new data constructor.

RAE: I think Tickish is the wrong way to go here, as that's all about performance measurements. Instead, given that addGroupFinalizer is always done from a splice, it seems you can put what you need into HsSplice. End RAE

  • When the type checker finds the annotation it writes to it the local typing environment:
    getLclEnv >>= writeIORef r . Just . tcl_env
    
  • When the finalizer runs, it reads its local typing environment from the mutable reference.

The local renamer environment should be copied as well, but that wouldn't change the discussion, I think.

Also, instead of mutable references we could generate integer keys and introduce a map of keys to local typing environments. RAE: I'm happy enough with mutable references here. End RAE

Last modified 18 months ago Last modified on Apr 19, 2016 5:46:00 PM