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<div name="messageBodySection" style="font-size: 14px; font-family: -apple-system, BlinkMacSystemFont, sans-serif;">Thanks for the explanation. But why did Haskell pick up the Reader monad in particular? Is this part of the HM type inference rule? </div>
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Regards,
<div>Qingbo Liu</div>
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On Dec 8, 2017, 13:12 -0500, David McBride <toad3k@gmail.com>, wrote:<br />
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<div>If you type :i Monad in ghci, you will see this instance<br />
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instance Monad ((->) r)<br />
<br /></div>
What this means is a function where the first argument is of type 'r'... is a monad. You can in fact use do notation / return on a tuple to manipulate its second argument monadically.<br />
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So let's look at what that does to the type signature of join when 'm' is ((->) b)<br />
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<div>join :: Monad m => m (m a) -> m a<br />
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-- m = ((->) b)<br />
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<div>join :: ((->) b ((->) b a)) -> (((->) b a))<br />
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Now we just have to move the arrows from prefix to infix. Let's do it step by step.<br />
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join :: ((->) b (b -> a)) -> (b -> a)<br />
join :: (b -> (b -> a)) -> (b -> a)<br />
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<div>x -> (y -> z) is equivalent to x -> y -> z<br /></div>
<div><br />
join :: (b -> b -> a) -> (b -> a)<br />
join :: (b -> b -> a) -> b -> a<br />
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<div>So now when you put an operator into it that takes two arguments<br />
<br /></div>
<div>(,) :: a -> b -> (a,b)<br />
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<div>You get the type you saw.<br /></div>
<div><br /></div>
<div>join (,) :: b -> (b, b)<br /></div>
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<div class="gmail_quote">On Fri, Dec 8, 2017 at 10:37 AM, Quentin Liu <span dir="ltr"><<a href="mailto:quentin.liu.0415@gmail.com" target="_blank">quentin.liu.0415@gmail.com</a>></span> wrote:<br />
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<div name="messageBodySection" style="font-size:14px;font-family:-apple-system,BlinkMacSystemFont,sans-serif">Hi,
<div><br /></div>
<div>The function `join` flattens a double-layered monad into one layer and its type signature is </div>
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<div> join :: (Monad m) => m (m a) -> m a</div>
<div><br /></div>
<div>But when the first argument supplied is `(,)`, the type signature becomes </div>
<div><br /></div>
<div> join (,) :: b -> (b, b)</div>
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<div>in ghci. The monad constraint is lost when supplied the first argument. So my question is why the type constraint is lost and what monad is supplied here.</div>
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<div name="messageSignatureSection" style="font-size:14px;font-family:-apple-system,BlinkMacSystemFont,sans-serif"><br />
Regards,
<div>Qingbo Liu</div>
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