How we can use type level string in template haskell?

Question

I played around with symbolVal and template haskell as follows on ghci.

>>> :set -XTemplateHaskell -XQuasiQuotes -XDataKinds
>>> import Language.Haskell.TH
>>> import Data.Proxy
>>> import GHC.TypeLits
>>> $([| symbolVal (Proxy :: Proxy "foo") |]) -- Code (1)
"foo"

>>> runQ [| symbolVal (Proxy :: Proxy "foo") |]
AppE (VarE GHC.TypeLits.symbolVal) (SigE (ConE Data.Proxy.Proxy) (AppT (ConT Data.Proxy.Proxy) (LitT (StrTyLit "foo"))))

>>> $(appE (varE 'symbolVal) (sigE (conE ''Proxy) (appT (conT ''Proxy) (litT (strTyLit "foo")))))

<interactive>:9:3: error:
    • Type constructor ‘Proxy’ used where a value identifier was expected
    • In the first argument of ‘symbolVal’, namely
        ‘Proxy :: Proxy "foo"’
      In the expression: (symbolVal (Proxy :: Proxy "foo"))
      In an equation for ‘it’: it = (symbolVal (Proxy :: Proxy "foo"))

Why this error occurs though Code (1) works well? How we can use type level string in template haskell?

I think the first `''Proxy` is supposed to be a data constructor, thus it should only have one tick: `'Proxy` — luqui, Nov 06 '16 at 08:42

score 0 · Answer 1 · answered May 10 '18 at 06:49

For completeness and future reference, I thought I would add the correct answer for reference.

λ> :set -XTemplateHaskell -XQuasiQuotes -XDataKinds
λ> import Language.Haskell.TH
λ> import Data.Proxy
λ> import GHC.TypeLits

λ> $([| symbolVal (Proxy :: Proxy "foo") |])
"foo"

λ> runQ [| symbolVal (Proxy :: 'Proxy "foo") |]
AppE (VarE GHC.TypeLits.symbolVal) (SigE (ConE Data.Proxy.Proxy) (AppT (PromotedT Data.Proxy.Proxy) (LitT (StrTyLit "foo"))))

λ> runQ [| symbolVal (Proxy :: Proxy "foo") |]
AppE (VarE GHC.TypeLits.symbolVal) (SigE (ConE Data.Proxy.Proxy) (AppT (ConT Data.Proxy.Proxy) (LitT (StrTyLit "foo"))))

λ> $(appE (varE 'symbolVal) (sigE (conE 'Proxy) (appT (conT ''Proxy) (litT (strTyLit "foo")))))
"foo"

How we can use type level string in template haskell?

1 Answers1