Is it possible to use SYB to transform the type?
-
12-11-2019 - |
Вопрос
I want to write a rename
function to replace String
names (which represent hierarchical identifiers) in my AST
with GUID
names (integers) from a symbol table carried as hidden state in a Renamer
monad.
I have an AST a
type that is parameterized over the type of name. Names in the leaves of the AST are of type Name a
:
data Name a = Name a
Which makes it easy to target them with a SYB transformer.
The parser is typed (ignoring the possibility of error for brevity):
parse :: String -> AST String
and I want the rename
function to be typed:
rename :: AST String -> Renamer (AST GUID)
Is it possible to use SYB to transform all Name String
's into Name GUID
's with a transformer:
resolveName :: Name String -> Renamer (Name GUID)
and all other values from c String
to c GUID
by transforming their children, and pasting them back together with the same constructor, albeit with a different type parameter?
The everywhereM
function is close to what I want, but it can only transform c a -> m (c a)
and not c a -> m (c b)
.
My fallback solution (other than writing the boiler-plate by hand) is to remove the type parameter from AST
, and define Name
like this:
data Name = StrName String
| GuidName GUID
so that the rename would be typed:
rename :: AST -> Renamer AST
making it work with everywhereM
. However, this would leave the possibility that an AST
could still hold StrName
's after being renamed. I wanted to use the type system to formally capture the fact that a renamed AST
can only hold GUID
names.
Нет правильного решения