Re: Understanding DsMeta module

But you need an expression that, say, produces a PlainTV. Where are you going to find an expression that does this without using the template-haskell library? On Dec 17, 2014, at 2:33 PM, Jan Stolarek wrote:

Thanks. That helps but I still don't understand why the calls are delegated to template-haskell library. Couldn't all of this be done locally?

Janek

Dnia środa, 17 grudnia 2014, Richard Eisenberg napisał:

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On Dec 17, 2014, at 12:29 PM, Jan Stolarek wrote:

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Why not implement repPlainTV like this: ?

repPlainTV :: Core TH.Name -> DsM (Core TH.TyVarBndr) repPlainTV (MkC nm) = return $ MkC (TH.PlainTV nm)

In short, that's ill typed. We have

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newtype Core a = MkC CoreExpr

The idea behind this type is that its (phantom) type parameter tracks the type of the expression stored within. Of course, the thing within is always just a core expression. TH.PlainTV takes a TH.Name and produces a TH.TyVarBndr. But, nm is a CoreExpr and MkC is expecting a CoreExpr, so your suggestion wouldn't type check.

The higher-level answer is that you're mixing levels. The goal in DsMeta is *not* to create the TH AST. It's to create *core expressions* that create the TH AST.

Does this help?

Richard