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| -rw-r--r-- | .gitignore | 10 | ||||
| -rw-r--r-- | LICENSE | 202 | ||||
| -rw-r--r-- | Setup.hs | 17 | ||||
| -rw-r--r-- | hierarchical-exceptions.cabal | 31 | ||||
| -rw-r--r-- | src/Control/Exception/Hierarchical.hs | 216 |
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diff --git a/.gitignore b/.gitignore new file mode 100644 index 0000000..d5e783d --- /dev/null +++ b/.gitignore @@ -0,0 +1,10 @@ +# .gitignore for hierarchical-exceptions -*- conf -*- +# This file is in the public domain. + +# Editor backup files +*~ +\#* +.\#* + +# Cabal +dist*/ @@ -0,0 +1,202 @@ + + Apache License + Version 2.0, January 2004 + http://www.apache.org/licenses/ + + TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION + + 1. Definitions. + + "License" shall mean the terms and conditions for use, reproduction, + and distribution as defined by Sections 1 through 9 of this document. + + "Licensor" shall mean the copyright owner or entity authorized by + the copyright owner that is granting the License. + + "Legal Entity" shall mean the union of the acting entity and all + other entities that control, are controlled by, or are under common + control with that entity. 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You may obtain a copy of the +License at + + http://www.apache.org/licenses/LICENSE-2.0 + +Unless required by applicable law or agreed to in writing, software distributed +under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR +CONDITIONS OF ANY KIND, either express or implied. See the License for the +specific language governing permissions and limitations under the License. -} + +import Distribution.Simple + +main = defaultMain diff --git a/hierarchical-exceptions.cabal b/hierarchical-exceptions.cabal new file mode 100644 index 0000000..a33ca69 --- /dev/null +++ b/hierarchical-exceptions.cabal @@ -0,0 +1,31 @@ +cabal-version: >=1.10 +name: hierarchical-exceptions +version: 1.0.0 +synopsis: Template Haskell functions to easily create exception + hierarchies +description: + Haskell supports exception hierarchies, but they require quite a bit of + boilerplate to declare. This package leverages Template Haskell to automate + declaration of large exception hierarchies. + . + This package requires GHC 7.6, as a bug in GHC 7.4 prevents expansion of + Template Haskell splices using 'deriving Typeable'. +license: Apache-2.0 +license-file: LICENSE +author: Benjamin Barenblat +maintainer: bbarenblat@galois.com +copyright: © 2014 Galois, Inc. +stability: provisional +category: Language +build-type: Simple +tested-with: GHC==7.6.3 + +library + hs-source-dirs: src + default-language: Haskell2010 + default-extensions: TemplateHaskell + other-extensions: DeriveDataTypeable, ExistentialQuantification + exposed-modules: Control.Exception.Hierarchical + build-depends: base >=4.6 && <5 + , template-haskell >= 2.2.0.0 && <3 + ghc-options: -Wall diff --git a/src/Control/Exception/Hierarchical.hs b/src/Control/Exception/Hierarchical.hs new file mode 100644 index 0000000..f906270 --- /dev/null +++ b/src/Control/Exception/Hierarchical.hs @@ -0,0 +1,216 @@ +{- Control.Exception.Hierarchical -- Template Haskell for defining exceptions +Copyright (C) 2014 Galois, Inc. + +Licensed under the Apache License, Version 2.0 (the "License"); you may not use +this file except in compliance with the License. You may obtain a copy of the +License at + + http://www.apache.org/licenses/LICENSE-2.0 + +Unless required by applicable law or agreed to in writing, software distributed +under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR +CONDITIONS OF ANY KIND, either express or implied. See the License for the +specific language governing permissions and limitations under the License. -} + +{-| "Control.Exception" leverages "Data.Typeable" to fake subtyping and thereby +give Haskell support for hierarchies of exceptions. However, defining +exception hierarchies requires quite a bit of boilerplate. For instance, to +define + + * a top-level exception, 'TracerException', + + * a sub-exception, 'TimingFailure', and + + * a sub-exception, 'WriteFailure', + +requires several paragraphs of code: + +> import Control.Exception +> import Data.Typeable (Typeable, cast) +> +> data TracerException = forall e. Exception e => TracerException e +> deriving Typeable +> +> instance Show TracerException where +> show (TracerException e) = show e +> +> instance Exception TracerException +> +> data TimingFailure = TimingFailure +> deriving (Show, Typeable) +> +> instance Exception TimingFailure where +> toException = toException . TracerException +> fromException x = do +> TracerException a <- fromException x +> cast a +> +> data WriteFailure = WriteFailure +> deriving (Show, Typeable) +> +> instance Exception WriteFailure where +> toException = toException . TracerException +> fromException x = do +> TracerException a <- fromException x +> cast a + +Instead of writing this, one could simply write + +> import Control.Exception (SomeException(SomeException)) +> import Control.Exception.Hierarchical +> +> mkAbstractException 'SomeException "TracerException" +> mkException 'TracerException "TimingFailure" +> mkException 'TracerException "WriteFailure" + +and allow Template Haskell to fill in the rest. + +This libray deals with two types of exceptions: /abstract/ and /concrete/ +exceptions. Both types can be caught with 'Control.Exception.catch' and other +associated functions; however, only you may only extend abstract exceptions, +and you may only throw concrete ones. This is a fundamental limitation of the +Haskell exception hierarchy system as it currently exists. -} + +module Control.Exception.Hierarchical + ( mkAbstractException + , mkException + ) where + +import Control.Exception (Exception(toException, fromException), + SomeException(SomeException)) +import Control.Monad ((>=>)) +import Data.Typeable (Typeable, cast) +import Language.Haskell.TH + + +--------------------------- Hierarchies and casting ---------------------------- + +{-| Creates declarations to make a data type a sub-exception of another +exception. This is best illustrated by some examples: + +> exceptionDeclaration 'SomeException 'MyAbstractException +> ======> +> instance Exception MyAbstractException + +> exceptionDeclaration 'MyAbstractException 'MyConcreteException +> ======> +> instance Exception MyConcreteException where +> toException = toException . MyConcreteException +> fromException = fromException >=> \(MyAbstractException x) -> cast x + +Note that exceptions directly under 'SomeException' are special-cased; the +default implementation for the 'Exception' type class is sufficient in this +case. -} +exceptionDeclaration :: Name -- ^ the name of the super-exception + -> Name -- ^ the name of the sub-exception + -> Dec +exceptionDeclaration super name = + InstanceD [] + (AppT (ConT ''Exception) (ConT name)) + (if super == 'SomeException + then + {- 'name' is directly under 'SomeException', so use the default + implementation for the conversion functions. -} + [] + else + {- 'name' is directly under some other exception, so explicitly + define the conversion functions to set up the hierarchy + correctly. -} + exceptionHierarchyFunctions super) + +{-| Creates declarations to implement the 'Exception' instance for a +sub-exception. -} +exceptionHierarchyFunctions :: Name -- ^ the name of the super-exception + -> [Dec] +exceptionHierarchyFunctions super = + let x = mkName "x" in + [ -- toException + ValD (VarP 'toException) + (NormalB (InfixE (Just (VarE 'toException)) + (VarE '(.)) + (Just (ConE super)))) + [] + -- fromException + , ValD (VarP 'fromException) + (NormalB (InfixE (Just (VarE 'fromException)) + (VarE '(>=>)) + (Just (LamE [ConP super [VarP x]] + (AppE (VarE 'cast) (VarE x)))))) + [] ] + + + +----------------------------- Abstract exceptions ------------------------------ + +{-| Creates an /abstract/ sub-exception of an existing exception. As discussed +in the introduction, such an exception cannot be thrown; it can only be +extended. -} +mkAbstractException :: Name + -- ^ the name of the super-exception’s data constructor + -> String -- ^ the name of the exception to create + -> Q [Dec] +mkAbstractException super name = do + let name' = mkName name + return [ abstractDataDeclaration name' + , abstractShowDeclaration name' + , exceptionDeclaration super name' ] + +{-| Defines a new data type suitable for use as an abstract exception. For +example, + +> abstractDataDeclaration (mkName "Name") +> ======> +> data Name = forall e. Exception e => Name e +> deriving Typeable -} +abstractDataDeclaration :: Name -> Dec +abstractDataDeclaration name = + let e = mkName "e" in + DataD [] name [] + [ForallC [PlainTV e] + [ClassP ''Exception [VarT e]] + (NormalC name [(NotStrict, VarT e)])] + [''Typeable] + +{-| Creates an instance declaration for an abstract exception type. For +example, + +> abstractShowDeclaration ''Name +> ======> +> instance Show Name where +> show (Name e) = show e -} +abstractShowDeclaration :: Name -> Dec +abstractShowDeclaration name = + let e = mkName "e" in + InstanceD [] + (AppT (ConT ''Show) (ConT name)) + [FunD 'show [Clause [ConP name [VarP e]] + (NormalB (AppE (VarE 'show) (VarE e))) + []]] + + +----------------------------- Concrete exceptions ----------------------------- + +{-| Creates a /concrete/ sub-exception of an existing exception. As discussed +in the introduction, such an exception cannot be extended; it can only be +thrown. -} +mkException :: Name + -- ^ the name of the super-exception’s data constructor + -> String -- ^ the name of the exception to create + -> Q [Dec] +mkException super name = do + let name' = mkName name + return [ dataDeclaration name' + , exceptionDeclaration super name' ] + +{-| Defines a new data type suitable for use as a concrete exception. For +example, + +> dataDeclaration (mkName "Name") +> ======> +> data Name = Name +> deriving (Show, Typeable) -} +dataDeclaration :: Name -> Dec +dataDeclaration name = + DataD [] name [] + [NormalC name []] + [''Show, ''Typeable] |