Hi.. I'm still struggling with monads ;-)
In the tutorial "All about monads" there is the function getOne used:
getOne:: (Random a) => (a,a) -> State StdGen a
getOne bound = do g <-get
(x,g')= return $ randomR bounds g
put g'
return x
In the haskell "Yet another Haskell tutorial" by Hal Daume it is said
that
do {fp <- e; es}
is translated to:
let ok p = do {es} ;
ok _ = fail "..."
in e >>= ok
Using this rule I was able to expand the first "<-" of getOne:
getOne2 :: (Random a) => (a,a) -> State StdGen a
getOne2 bounds = let ok g = do (x,g') <- return $ randomR bounds g
put g'
return x
ok _ = fail "asd"
in get >>= ok
but I couldn't manage to also expand the second.
My attempt is:
getOne3 :: (Random a) => (a,a) -> State StdGen a
getOne3 bounds = let ok g = do let ok2 (x,g') = do { put g'; return x } --
ok2 _ = fail "2"
in (return $ randomR bounds g >>= ok2
ok _ = fail "1"
in get >>= ok
But I get this compile error in at the line marked by a trailing --:
"The last statement in a 'do' construct must be an expression"
Can you help me?
Sincerly Marc
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