Why is EvTerm limited?

Simon Peyton Jones simonpj at microsoft.com
Fri Jan 19 17:05:06 UTC 2018

|  What would break if we had
|   | EvExpr CoreExpr
|  as an additional constructor there?

This has come up before.  I think that'd be a solid win. 

In fact, eliminate all the existing evidence constructors with "smart constructors" that produce an EvExpr.  That'd mean moving stuff from the desugarer into these smart constructors, but that's ok.

I /think/ I didn't do that initially only because there were very few forms and it mean that there was no CoreExpr stuff in the type checker. But as we add more forms that decision looks and less good.

You'd need to add zonkCoreExpr in place of zonkEvTerm.

evVarsOfTerm is called quite a bit; you might want to cache the result in the EvExpr constructor.

Make a ticket and execute?


|  -----Original Message-----
|  From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Joachim Breitner
|  Sent: 19 January 2018 14:01
|  To: Glasgow-Haskell-Users users <glasgow-haskell-users at haskell.org>
|  Subject: Why is EvTerm limited?
|  Hi,
|  I had some funky idea where a type checker plugin would have to
|  synthesize code for a custom-solved instances on the fly. But it seems
|  that does not work because EvTerm is less expressive than Core
|  (especially, no lambdas):
|  https://na01.safelinks.protection.outlook.com/?url=https:%2F%2Fdownloa
|  ds.haskell.org%2F~ghc%2F8.2.2%2Fdocs%2Fhtml%2Flibraries%2Fghc-
|  8.2.2%2FTcEvidence.html%23t:EvTerm&data=02%7C01%7Csimonpj%40microsoft.
|  com%7C513ff7ae83914913225008d55f452dec%7C72f988bf86f141af91ab2d7cd011d
|  b47%7C1%7C0%7C636519673089385423&sdata=kFkUugVn02Nfu4QXJ6dkVwtx8KWFrTM
|  fWcVEiwf6KyI%3D&reserved=0
|  What would break if we had
|   | EvExpr CoreExpr
|  as an additional constructor there?
|  Cheers,
|  Joachim
|  --
|  Joachim “nomeata” Breitner
|    mail at joachim-breitner.de
|  https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.j
|  oachim-
|  breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7C513ff7ae839149
|  13225008d55f452dec%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636519
|  673089385423&sdata=Vh4BvbeEVUBIntKcf3XEseOzwUTx2RHPuANTY328dpM%3D&rese
|  rved=0

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