data NewTy(<params>) = NewCon(WrappedTy(<params>))
if there is an instance Class(WrappedTy(<params>)), a Class instance for NewTy(<params>) can be automatically derived. This PR implements automatic instance derivation with the syntax
deriving instance Class(NewTy(<params>)) <givens>
Note that automatic instance derivation is asked for in issue #945, but the approach taken in this PR differs from extending DictExpr n with a new contructor NewtypeDict (DictType n) (DictExpr n), as suggested in issue #945. Instead, this PR synthesises an InstanceDef for Class(NewTy(<params>)) from the definition of instance Class(WrappedTy(<params>)).
The main difficulty in synthesising the InstanceDef for Class(NewTy(<params>)) is converting the methods of instance Class(WrappedTy(<params>)), which operate on WrappedTy(<params>), to methods that work with NewTy(<params>). Since the types WrappedTy(<params>) and NewTy(<params>) are isomorphic, this conversion is generally possible. Note that in order to facilitate this conversion, an isomorphism between WrappedTy(<params>) and NewTy(<params>) must be exhibited, consisting of a pair of maps, namely of a (forward-directed) isomorphism and its (backward-directed) inverse.
For types of the form
if there is an
instance Class(WrappedTy(<params>)), aClassinstance forNewTy(<params>)can be automatically derived. This PR implements automatic instance derivation with the syntaxNote that automatic instance derivation is asked for in issue #945, but the approach taken in this PR differs from extending
DictExpr nwith a new contructorNewtypeDict (DictType n) (DictExpr n), as suggested in issue #945. Instead, this PR synthesises anInstanceDefforClass(NewTy(<params>))from the definition ofinstance Class(WrappedTy(<params>)).The main difficulty in synthesising the
InstanceDefforClass(NewTy(<params>))is converting the methods ofinstance Class(WrappedTy(<params>)), which operate onWrappedTy(<params>), to methods that work withNewTy(<params>). Since the typesWrappedTy(<params>)andNewTy(<params>)are isomorphic, this conversion is generally possible. Note that in order to facilitate this conversion, an isomorphism betweenWrappedTy(<params>)andNewTy(<params>)must be exhibited, consisting of a pair of maps, namely of a (forward-directed) isomorphism and its (backward-directed) inverse.