id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,testcase,blockedby,blocking,related,differential,wikipage
7828,RebindableSyntax and Arrow,AlessandroVermeulen,,"When trying to add constraints to the types of the arrow primitives I get a type error. I think that doing such a thing should be possible and I've attached the code I used to test this. The errors I get when using the arrow notation for the function test are as follows:
{{{
test :: Typeable a => R a a
test = proc n -> returnA -< n
}}}
{{{
bug-arrow.hs:15:8:
Could not deduce (Typeable c) arising from a use of `arr'
from the context (Typeable a)
bound by the type signature for test :: Typeable a => R a a
at bug-arrow.hs:14:9-27
Possible fix:
add (Typeable c) to the context of
a type expected by the context: (b -> c) -> R b c
or the type signature for test :: Typeable a => R a a
In the expression: arr
When checking that `arr' (needed by a syntactic construct)
has the required type: forall b1 c1. (b1 -> c1) -> R b1 c1
arising from a proc expression at bug-arrow.hs:15:8-29
In the expression: proc n -> returnA -< n
bug-arrow.hs:15:8:
Could not deduce (Typeable c) arising from a use of `>>>'
from the context (Typeable a)
bound by the type signature for test :: Typeable a => R a a
at bug-arrow.hs:14:9-27
Possible fix:
add (Typeable c) to the context of
a type expected by the context: R a1 b -> R b c -> R a1 c
or the type signature for test :: Typeable a => R a a
In the expression: (>>>)
When checking that `(>>>)' (needed by a syntactic construct)
has the required type: forall a2 b1 c1.
R a2 b1 -> R b1 c1 -> R a2 c1
arising from a proc expression at bug-arrow.hs:15:8-29
In the expression: proc n -> returnA -< n
bug-arrow.hs:15:8:
Could not deduce (Typeable d) arising from a use of `first'
from the context (Typeable a)
bound by the type signature for test :: Typeable a => R a a
at bug-arrow.hs:14:9-27
Possible fix:
add (Typeable d) to the context of
a type expected by the context: R b c -> R (b, d) (c, d)
or the type signature for test :: Typeable a => R a a
In the expression: first
When checking that `first' (needed by a syntactic construct)
has the required type: forall b1 c1 d1.
R b1 c1 -> R (b1, d1) (c1, d1)
arising from a proc expression at bug-arrow.hs:15:8-29
In the expression: proc n -> returnA -< n
}}}
When I replace the definition with the translated core code (minus type applications and scoped type variables) the code compiles:
{{{
test :: Typeable a => R a a
test =
(>>>)
(arr (\ (n_apd) -> n_apd))
((>>>)
(arr (\ (ds_dst) -> ds_dst))
(returnA)
)
}}}
",bug,new,normal,,Compiler (Type checker),7.6.2,,"Arrows, RebindableSyntax",ross@…,Unknown/Multiple,Unknown/Multiple,GHC rejects valid program,,,,"#1537, #3613",Phab:D72,