Re: question about GHC API on GHC plugin

Ahh... I get it now! Thanks for your patience :) I have a new question: I'm working on supporting literals now. I'm having trouble creating something that looks like `(App (Var F#) (Lit 1.0))` because I don't know how to create a variable that corresponds to the `F#` constructor. The mkWiredInName function looks promising, but overly complicated. Is this the correct function? If so, what do I pass in for the Module, Unique, TyThing, and BuiltInSyntax parameters? On Thu, Aug 20, 2015 at 5:55 PM, Andrew Farmer wrote:

The `buildDictionary` function takes a Var with a dictionary type, and builds the expression which implements that dictionary.

For instance, you might create a new Var:

x :: Num Float

and pass that to buildDictionary. It will return:

(x, [NonRec x $fNumFloat])

which you could blindly turn into:

let x = $fNumFloat in x

or you could do what buildDictionaryT (a bit further down in the same module), and spot that case and just return $fNumFloat directly. (The list can have more than one element in the case that dictionaries are built in terms of other dictionaries.)

Thus, you've built a dictionary expression of type Num Float.

As I understand it, you want to pass something 'log' and get back the dictionary argument. You'll need to choose a type (like Float), but once that is done, it should be easy to use buildDictionary to build the dictionary arguments... just take apart the type of 'log @ Float', make a new Var with the argument type, build a dictionary expression, and apply it.

On Thu, Aug 20, 2015 at 5:05 PM, Mike Izbicki wrote:

...

I'm pretty sure the `buildDictionary` function doesn't do what I need. AFAICT, you pass it a `Var` which contains a dictionary, and it tells you what is in that dictionary. What I need is a function with type `Var -> Var` where the first `Var` contains a function, and the output `Var` is the dictionary.

For example, given the expression:

log (a1+a2)

In core, this might look like:

log @ Float $fFloatingFloat (+ @ Float $fNumFloat a1 a2)

I want to mechanically construct the core code above. When doing so, each function within a type class has an extra argument, which is the dictionary for that type class. `log` no longer takes one parameter; in core, it takes two. I'm having trouble figuring out how to get the appropriate dictionary to pass as the "dictionary parameter" to these functions.

On Mon, Aug 17, 2015 at 4:21 PM, Andrew Farmer wrote:

...

HERMIT has some code for building dictionaries for a given predicate type (by invoking the typechecker functions that do this):

https://github.com/ku-fpg/hermit/blob/master/src/HERMIT/Dictionary/GHC.hs#L2...

The functions to run TcM computations inside CoreM are here:

https://github.com/ku-fpg/hermit/blob/master/src/HERMIT/Monad.hs#L242 and https://github.com/ku-fpg/hermit/blob/master/src/HERMIT/GHC/Typechecker.hs#L...

Perhaps that will help get you started?

I would like to push these interfaces back into the GHC API at some point, but just haven't done it yet.

HTH Andrew

On Mon, Aug 17, 2015 at 4:12 PM, Mike Izbicki wrote:

...

I'm not sure how either of those two functions can help me. The problem is that given an operator (e.g. `+`), I don't know the name of the dictionary that needs to be passed in as the first argument to the operator. I could probably hard code these names, but then the plugin wouldn't be able to work with alternative preludes.

On Fri, Aug 7, 2015 at 11:20 PM, Edward Z. Yang wrote:

...

Hello Mike,

Give importDecl from LoadIface a try, or maybe tcLookupGlobal if you're in TcM.

Edward

Excerpts from Mike Izbicki's message of 2015-08-07 15:40:30 -0700:

...

I'm trying to write a GHC plugin. The purpose of the plugin is to provide Haskell bindings to Herbie. Herbie (https://github.com/uwplse/herbie) is a program that takes a mathematical statement as input, and gives you a numerically stable formula to compute it as output. The plugin is supposed to automate this process for Haskell programs.

I can convert the core expressions into a format for Herbie just fine. Where I'm having trouble is converting the output from Herbie back into core. Given a string that represents a numeric operator (e.g. "log" or "+"), I can get that converted into a Name that matches the Name of the version of that operator in scope at the location. But in order to create an Expr, I need to convert the Name into a Var. All the functions that I can find for this (e.g. mkGlobalVar) also require the type of the variable. But I can't find a way to figure out the Type given a Name. How can I do this?

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