[GHC] #14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument
#14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler | Version: 8.4.1-alpha1 Keywords: deriving | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Running this program in GHC 8.4.1-alpha: {{{#!hs {-# LANGUAGE EmptyDataDeriving #-} {-# OPTIONS_GHC -ddump-deriv #-} module Main (main) where data Empty deriving Show loop :: Empty loop = let x = x in x main :: IO () main = print loop }}} One would expect this to loop infinitely at runtime, but in practice, that is not the case: {{{ $ /opt/ghc/8.4.1/bin/runghc Bug.hs ==================== Derived instances ==================== Derived class instances: instance GHC.Show.Show Main.Empty where GHC.Show.showsPrec z_a1Iu = case z_a1Iu of Derived type family instances: ==================== Filling in method body ==================== GHC.Show.Show [Main.Empty] GHC.Show.show = GHC.Show.$dmshow @(Main.Empty) ==================== Filling in method body ==================== GHC.Show.Show [Main.Empty] GHC.Show.showList = GHC.Show.$dmshowList @(Main.Empty) Bug.hs: Bug.hs:5:21-24: Non-exhaustive patterns in case }}} The `-ddump-deriv` output reveals why: the `showsPrec` implementation for `Empty` is casing on the //precedence// argument, not the actual value of type `Empty`! This results in the non-exhaustive patterns error. This is my fault, so I'll prepare a fix :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14692 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: high | Milestone: 8.4.1 Component: Compiler | Version: 8.4.1-alpha1 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4328 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D4328 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14692#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument
-------------------------------------+-------------------------------------
Reporter: RyanGlScott | Owner: (none)
Type: bug | Status: patch
Priority: high | Milestone: 8.4.1
Component: Compiler | Version: 8.4.1-alpha1
Resolution: | Keywords: deriving
Operating System: Unknown/Multiple | Architecture:
Type of failure: Incorrect result | Unknown/Multiple
at runtime | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4328
Wiki Page: |
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Comment (by Ben Gamari
#14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: high | Milestone: 8.4.1 Component: Compiler | Version: 8.4.1-alpha1 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4328 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: patch => closed * resolution: => fixed Comment: Merged with 1d05e0c74ad861946e4deb4f27a77b181cab3089. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14692#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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