[Haskell-cafe] let vs do?
stefan at cs.uu.nl
Fri Jun 29 00:49:09 EDT 2007
> 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
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
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