CODEWITHSUNDEEP
haskellghcghc-api
aleator
aleator

Reputation: 4486

GHC-api and typechecking class constraints

I am trying to build a simple ghci-like console using ghc-api. I've gotten to a point where I can extract Type's of expressions using exprType and to evaluate them. Is there also an easy way to check if the type of the expression has an instance of a given class?

Edit: It seems that functions I'm looking for would require an InstEnv, but I have no clue where to find that.

Upvotes: 4

Views: 171

Answers (1)

David Miani
David Miani

Reputation: 14678

The isInstance method may do what you require.

Example in ghci:

> :set -XTemplateHaskell
> import Language.Haskell.TH
> $(stringE . show =<< (isInstance ''Functor . (: []) =<< [t| [] |]))
"True"
> $(stringE . show =<< (isInstance ''Show . (: []) =<< [t| Maybe Int |]))
"True"
$(stringE . show =<< (isInstance ''Show . (: []) =<< [t| (Int -> Bool) |]))
"False"

Its type sig is 

isInstance :: Name -> [Type] -> Q Bool

Ie you give a name (retrieved using '' or with the mkName function) for the class, then you pass the types to check against the class (more than one will be required if the class is a multiparam type class). It will then return True or False in the Q monad.

Upvotes: 3

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