#836: rebindable if-then-else syntax
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Reporter: nibro | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: _|_
Component: Compiler (Parser) | Version: 6.13
Keywords: | Testcase: N/A
Blockedby: | Difficulty: Unknown
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by vivian):
I think of 'if then else' as less fundamental than 'case' and so the last
point seems to not be a problem.
If "if then else" is bound to '''the''' in scope variable 'cond' then 'if
then else' can't be used in two different ways in the same module, which
is why I suggested class overloading, which can select the appropriate
instance from the return type in the context.
LogicT defines a 'soft cut', which is similar to
{{{
condition :: (Error e, Monad m)
=> ErrorT e m a -> (b -> ErrorT e m a) -> ErrorT e m a -> Error
T e m a
condition e1 e2 e3 = ErrorT $ do
b <- runErrrorT e1
case b of
Left _ -> runErrorT e3
Right b' -> runErrorT (e2 b')
}}}
and it would nice to be able to rebind this to "if then else"
A contrived example:
{{{
loadDefaults filename = if (openFile filename)
then (\handle -> do
version <- loadConfiguration
handle
if version < 2
then addNewFeatures
else $ return ())
else defaultConfiguration
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/836#comment:7>
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