diff options
| -rw-r--r-- | libraries/template-haskell/Language/Haskell/TH/Syntax.hs | 78 |
1 files changed, 41 insertions, 37 deletions
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs index 8398bafd53..513edd5abc 100644 --- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs +++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs @@ -335,39 +335,9 @@ type role TExp nominal -- See Note [Role of TExp] newtype TExp (a :: TYPE (r :: RuntimeRep)) = TExp { unType :: Exp -- ^ Underlying untyped Template Haskell expression } --- ^ Represents an expression which has type @a@. Built on top of 'Exp', typed --- expressions allow for type-safe splicing via: --- --- - typed quotes, written as @[|| ... ||]@ where @...@ is an expression; if --- that expression has type @a@, then the quotation has type --- @'Q' ('TExp' a)@ --- --- - typed splices inside of typed quotes, written as @$$(...)@ where @...@ --- is an arbitrary expression of type @'Q' ('TExp' a)@ --- --- Traditional expression quotes and splices let us construct ill-typed --- expressions: --- --- >>> fmap ppr $ runQ [| True == $( [| "foo" |] ) |] --- GHC.Types.True GHC.Classes.== "foo" --- >>> GHC.Types.True GHC.Classes.== "foo" --- <interactive> error: --- • Couldn't match expected type ‘Bool’ with actual type ‘[Char]’ --- • In the second argument of ‘(==)’, namely ‘"foo"’ --- In the expression: True == "foo" --- In an equation for ‘it’: it = True == "foo" +-- ^ Typed wrapper around an 'Exp'. -- --- With typed expressions, the type error occurs when /constructing/ the --- Template Haskell expression: --- --- >>> fmap ppr $ runQ [|| True == $$( [|| "foo" ||] ) ||] --- <interactive> error: --- • Couldn't match type ‘[Char]’ with ‘Bool’ --- Expected type: Q (TExp Bool) --- Actual type: Q (TExp [Char]) --- • In the Template Haskell quotation [|| "foo" ||] --- In the expression: [|| "foo" ||] --- In the Template Haskell splice $$([|| "foo" ||]) +-- This is the typed representation of terms produced by typed quotes. -- -- Representation-polymorphic since /template-haskell-2.16.0.0/. @@ -395,13 +365,13 @@ unsafeTExpCoerce m = do { e <- m TExp's argument must have a nominal role, not phantom as would be inferred (#8459). Consider - e :: TExp Age - e = MkAge 3 + e :: Code Q Age + e = [|| MkAge 3 ||] foo = $(coerce e) + 4::Int The splice will evaluate to (MkAge 3) and you can't add that to -4::Int. So you can't coerce a (TExp Age) to a (TExp Int). -} +4::Int. So you can't coerce a (Code Q Age) to a (Code Q Int). -} -- Code constructor @@ -409,6 +379,40 @@ type role Code representational nominal -- See Note [Role of TExp] newtype Code m (a :: TYPE (r :: RuntimeRep)) = Code { examineCode :: m (TExp a) -- ^ Underlying monadic value } +-- ^ Represents an expression which has type @a@, built in monadic context @m@. Built on top of 'TExp', typed +-- expressions allow for type-safe splicing via: +-- +-- - typed quotes, written as @[|| ... ||]@ where @...@ is an expression; if +-- that expression has type @a@, then the quotation has type +-- @Quote m => Code m a@ +-- +-- - typed splices inside of typed quotes, written as @$$(...)@ where @...@ +-- is an arbitrary expression of type @Quote m => Code m a@ +-- +-- Traditional expression quotes and splices let us construct ill-typed +-- expressions: +-- +-- >>> fmap ppr $ runQ (unTypeCode [| True == $( [| "foo" |] ) |]) +-- GHC.Types.True GHC.Classes.== "foo" +-- >>> GHC.Types.True GHC.Classes.== "foo" +-- <interactive> error: +-- • Couldn't match expected type ‘Bool’ with actual type ‘[Char]’ +-- • In the second argument of ‘(==)’, namely ‘"foo"’ +-- In the expression: True == "foo" +-- In an equation for ‘it’: it = True == "foo" +-- +-- With typed expressions, the type error occurs when /constructing/ the +-- Template Haskell expression: +-- +-- >>> fmap ppr $ runQ (unTypeCode [|| True == $$( [|| "foo" ||] ) ||]) +-- <interactive> error: +-- • Couldn't match type ‘[Char]’ with ‘Bool’ +-- Expected type: Code Q Bool +-- Actual type: Code Q [Char] +-- • In the Template Haskell quotation [|| "foo" ||] +-- In the expression: [|| "foo" ||] +-- In the Template Haskell splice $$([|| "foo" ||]) + -- | Unsafely convert an untyped code representation into a typed code -- representation. @@ -958,7 +962,7 @@ sequenceQ = sequence -- Haskell quotation is bound outside the Oxford brackets (@[| ... |]@ or -- @[|| ... ||]@) but not at the top level. As an example: -- --- > add1 :: Int -> Q (TExp Int) +-- > add1 :: Int -> Code Q Int -- > add1 x = [|| x + 1 ||] -- -- Template Haskell has no way of knowing what value @x@ will take on at @@ -966,7 +970,7 @@ sequenceQ = sequence -- -- A 'Lift' instance must satisfy @$(lift x) ≡ x@ and @$$(liftTyped x) ≡ x@ -- for all @x@, where @$(...)@ and @$$(...)@ are Template Haskell splices. --- It is additionally expected that @'lift' x ≡ 'unTypeQ' ('liftTyped' x)@. +-- It is additionally expected that @'lift' x ≡ 'unTypeCode' ('liftTyped' x)@. -- -- 'Lift' instances can be derived automatically by use of the @-XDeriveLift@ -- GHC language extension: |