Thomas,
let x = ... in ...
is only equal
do x <- ...; ...
in the Identity monad. Also, why would "do" be more primitive than
"let". That way you would have to use monads everywhere. Also,
let is treated specially by the type checker (IIRC) and there are
many, many other reasons not to do that.
As you already hinted at in a later message, this has to do with let-
bindings being potentially polymorphic and monadic bindings being
necessarily monomorphic:
import Control.Monad.Identity
foo = let id = \x -> x in (id 'x',
id 42) -- well-typed
bar = runIdentity $ do id <- return (\x -> x) ; return (id 'x',
id 42) -- ill-typed
Cheers,
Stefan
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