#4316: Interactive "do" notation in GHCi
---------------------------------+------------------------------------------
Reporter: mitar | Owner:
Type: feature request | Status: new
Priority: normal | Component: GHCi
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by vivian):
Replying to [comment:3 mitar]:
> Replying to [comment:2 vivian]:
> Wouldn't it be better if things would be interpreted immediately?
This would require GHCi to somehow know how to run arbitrary monads. This
is a problem as each monad has a datatype-specific run function. Some
monads return {{{Value,State}}} (StateT), others return {{{Either Error
Value}}} (ErrorT), and so on.
{{{
Prelude> flip evalStateT 1 $ do
StateT Int IO a | i <- get
StateT Int IO a | lift $ putStrLn $ show i
1
}}}
somehow insert
{{{
flip runStateT <<<missing parameter>>> $ do
}}}
before the next line:
{{{
StateT Int IO a | return $ i + 1
StateT Int IO a |
2
Prelude>
}}}
The difference is between "prompting for more input," which relies only on
the original function call {{{flip evalStateT}}}, a 'syntactic' operation,
and multiple evaluations, a 'semantic' operation.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4316#comment:4>
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