exception-via
DerivingVia for your hierarchical exceptions
https://github.com/parsonsmatt/exception-via#readme
Version on this page: | 0.1.0.0 |
LTS Haskell 22.38: | 0.2.0.0 |
Stackage Nightly 2023-12-26: | 0.2.0.0 |
Latest on Hackage: | 0.2.0.0 |
exception-via-0.1.0.0@sha256:159cbd75cc2f2b54bf2824086dc7e81e5d3f8861a8f6254c8f1fda8a7e163657,902
Module documentation for 0.1.0.0
exception-via
Haskell supports hierarchical exceptions, but there’s a bit of boilerplate involved.
The documentation for Control.Exception
has a write-up:
---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
instance Show SomeCompilerException where
show (SomeCompilerException e) = show e
instance Exception SomeCompilerException
compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException
compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
SomeCompilerException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
instance Show SomeFrontendException where
show (SomeFrontendException e) = show e
instance Exception SomeFrontendException where
toException = compilerExceptionToException
fromException = compilerExceptionFromException
frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException
frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
SomeFrontendException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception
data MismatchedParentheses = MismatchedParentheses
deriving Show
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromException
Woof! That’s a lot of code just to have nested exceptions. Especially since Java devs can just write
public class CompilerException extends Exception { ... }
public class FrontendException extends CompilerException {....}
This library attempts to help by providing a newtype
wrapper you can use with DerivingVia
.
With basic exceptions, you don’t need this - the default methods on Exception
default to a top-level exception.
data EasyException = EasyException
deriving stock Show
deriving anyclass Exception
Let’s make those nested exceptions.
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
deriving stock instance Show SomeCompilerException
deriving anyclass instance Exception SomeCompilerException
instance Hierarchy SomeCompilerException where
toParent = SomeCompilerException
fromParent (SomeCompilerException e) = cast e
The Hierarchy
class is required to tell us how to unpack and pack things in the exception type.
Now let’s get to the frontend exception.
It’s a subtype of SomeCompilerException
, so we’ll derive the Exception
instance using our via
-type.
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
deriving stock instance Show SomeFrontendException
deriving
via (SomeFrontendException <!!! SomeCompilerException)
instance Exception SomeFrontendException
That’s it.
We need to define the Hierarchy
instance, which is extremely boilerplate:
instance Hierarchy SomeFrontendException where
toParent = SomeFrontendException
fromParent (SomeFrontendException e) = cast e
A TemplateHaskell
helper would be nice…
mkHierarchy ''SomeFrontendException
Much better.
And now we have our actual exception types:
data MismatchedParentheses = MismatchedParentheses
deriving stock Show
deriving
via (MismatchedParentheses <!!! SomeFrontendException)
Exception MismatchedParentheses
Easy peasy.