Hi ghc-devs,

I've been trying to implementing a function with the signature:

elaborateExpr :: GhcMonad m => String -> m CoreExpr

It should take a string of the form:

"\x y -> x <> y :: Semigroup a => a -> a -> a"

and give back a core expression with the dictionaries filled in:

\ ($dSem :: Semigroup a) (x :: a) (y :: a) -> (<>) $dSem x y

The goal is to use the function to add elaboration support for liquidhaskell.

I looked into the implementation of exprType and defined my own elaborateExpr similarly by calling desugarExpr on the expression (which has type LHsExpr GhcTcId) returned by tcInferSigma.

GhcTcId is a synonym of GhcTc so the program I wrote typechecks, but it's definitely not right. The elaborateExpr function I defined would return something even when the expression doesn't typecheck, or occasionally give a runtime exception:

ghc-elaboration-test: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-unknown-linux):
    dsEvBinds

I must have broken some invariants somehow.

What is the correct way of defining such a function (takes a string and returns a CoreExpr)? It appears to me that I should convert LHsExpr GhcPs into LHsExpr GhcTc first before calling deSugarExpr, but I don't know how.

Thank you,

- Yiyun