Opened 4 years ago

Last modified 21 months ago

#8258 new bug

GHC accepts `data Foo where` in H2010 mode

Reported by: hvr Owned by:
Priority: low Milestone:
Component: Compiler (Parser) Version: 7.0.4
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: GHC accepts invalid program Test Case:
Blocked By: Blocking: #11384
Related Tickets: Differential Rev(s):
Wiki Page:


According to the Haskell2010 report,

The grammar productions relevant for data declarations are the following:

topdecl	→ data [context =>] simpletype [= constrs] [deriving]

(from 4.2.1 Algebraic Datatype Declarations)

However, GHC 7.0.4/7.4.2/7.6.3/HEAD all happily compile the following Haskell module without any warning when using -XHaskell2010 -Wall, even though this is not valid Haskell2010 afaik:

module Foo where

data Foo where

The trailing where in the data declaration is not valid Haskell2010 (without language extensions such as GADT).

At the very least, this divergence from the Haskell Report should be mentioned in 14.1.1. Divergence from Haskell 98 and Haskell 2010.

Change History (7)

comment:1 Changed 3 years ago by thoughtpolice


Moving to 7.10.1

comment:2 Changed 3 years ago by thoughtpolice


Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

comment:3 Changed 2 years ago by siddhanathan

Should a trailing where be considered legal?

For example, the following is valid:

c = 5 where

a = 5 where b = 5

And the code above will be parsed as:

==================== Parser ====================
c = 5
a = 5
      b = 5

But the trailing where after c = 5 was probably accidental.

comment:4 Changed 2 years ago by thomie

Component: CompilerCompiler (Parser)

c = 5 where is ok.


decl 	→ 	(funlhs | pat) rhs
rhs 	→ 	= exp [where decls]

decls 	→ 	{ decl1 ; … ; decln } 	    (n ≥ 0) 

comment:5 Changed 2 years ago by thoughtpolice


Milestone renamed

comment:6 Changed 21 months ago by thomie

Blocking: 11384 added

comment:7 Changed 21 months ago by thomie

Milestone: 8.0.1
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