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Last edited by Ben Gamari
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signature sections

The SignatureSections Language Extension

TODO forbid (:: forall a . a) (any variables must be in scope), mention wildcards, explain restrictions, typing-rule == type-checking the translation, ...

Semantics

The SignatureSections language pragma enables the use of the :: type-signature operator in "type-signature sections" expressions, i.e.

aexp → ( :: type )

where type must not have any free (not bound by outer context) type-variables.

Translation: The following identity holds

( :: type ) = \ x -> x :: type

Applications

Like Haskell2010 expression type-signatures, type-signature sections are useful to explicitly type expressions to help resolve ambiguous typing (e.g. due to overloading).

Examples

canonDouble :: String -> String
canonDouble = show . read                -- type error
canonDouble = show . (:: Double) . read  -- OK
typeRep :: proxy a -> TypeRep
data Proxy a = Proxy

-- without `SignatureSections`
a = typeRep (Proxy :: Proxy Bool)

-- with `SignatureSections
a = typeRep (:: Bool)

References