#7479: ArrowChoice unit law in haddock seems to be wrong
------------------------------+---------------------------------------------
Reporter: pminten | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.7 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Documentation bug | Blockedby:
Blocking: | Related:
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In the haddock documentation of !ArrowChoice there is an (unnamed) law:
"{{{left f >>> arr Left = arr Left >>> f}}}". In Ross Paterson's
[http://www.soi.city.ac.uk/~ross/papers/fop.html Arrows and Computations]
paper there is a similar law named the unit law for !ArrowChoice that goes
"{{{pure Left >>> left f = f >>> pure Left}}}" ("pure" in that paper is
"arr" in the base library).
Reordering the unit law from the paper and replacing "pure" with "arr" the
unit law from the paper is "{{{f >>> arr Left = arr Left >>> left f}}}"
where the law from haddock is "{{{left f >>> arr Left = arr Left >>>
f}}}". The laws are similar but "left" is used in different places.
The law from haddock appears to be invalid, there does not appear to be a
way to create an arrow "f" in such a way that it works with both sides of
the equation. This can be verified by typing the following lines in GHCi:
{{{
import Control.Arrow
let foo f = left f >>> arr Left
let bar f = arr Left >>> f
let same :: a b c -> a b c -> (); same _ _ = ()
\f -> same (foo f) (bar f)
}}}
The last line gives a type error.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7479>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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