Lookup rules associated with functions/values in GHCI

It would be nice to have a way to look up in GHCi what rules can fire for a certain constant as the outmost symbol on the LHS. For example, something like

ghci> :pragmas map
{-# RULE "map"         [~1] forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs) #-}
{-# RULE "map/coerce"  [1]               map coerce = coerce #-}

As the name suggests, I think we would also want to list applicable NOINLINE, INLINE, INLINABLE, CONLIKE, SPECIALIZE, etc pragmas too. This would simplify the process of debugging mis-firing rules.

If this isn't too difficult, I'd love to try implementing it. I've been looking for some way of contributing to GHC.

This initially came up here http://stackoverflow.com/q/38651602/3072788.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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changed weight to 5

added GHCi Tfeature request Trac import labels

I can see the value, and I don't think it'd be hard. Happy to advise.

First look at how :info works, I suggest.

Simon

Replying to [ticket:12895#comment:127878 simonpj]:

I can see the value, and I don't think it'd be hard. Happy to advise.

First look at how :info works, I suggest.

Simon

After a quick survey of how :info works, I think I'm starting to see what needs to be done (or getting lost). As :info does for finding instances, I'll need to start by loading some interfaces to make sure I really do find all my RULES, then I should be able to get out of the ExternalPackageState a RuleBase, at which point I just need to filter for the right CoreRules .

However, is CoreRule really the abstraction I'm looking for? I noticed there is also an interface level IfaceRule and a type checker level LRuleDecl Id (from TcGblEnv) so I thought I'd ask...

More generally, how little of this sounds right? :)

Sounds about right. You should think of IfaceSyn as simply a tree full of strings that is the result of parsing the bits in an interface file. It's not suitable for anythingn except printing.

TcIface converts IfaceSyn to CoreSyn, CoreRule, TyCon etc. I suggest you work with CoreRules.

Simon

added Pnormal label