<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">What is purpose formal, except to make heterogenous lists?<div class="">I find it not necessary in practice, exept to emulate OO </div><div class="">passing through lists, when you need to implement OO like</div><div class="">Behavior that is polymorphism. Those things come natural</div><div class="">in Haskell wihout forall .</div><div class=""><br class=""></div><div class="">Greetings, Branimir.<br class=""><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 19.09.2021., at 12:55, Kai-Oliver Prott <<a href="mailto:kai.prott@hotmail.de" class="">kai.prott@hotmail.de</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class=""><p class="">Some comments in-line :)<br class="">
Best, <br class="">
Kai Prott<br class="">
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<div class=""><font face="arial, sans-serif" class="">> <span style="font-size: 1em; white-space: pre-wrap;" class="">pattern Any2 :: forall . forall a. a -> Some</span></font></div>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">We're friends here. I think I can share that my reaction was a rather loud WTF??!!??</font></pre>
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Yep, that is quite weird. <br class="">
<blockquote type="cite" cite="mid:CABU_mxinuS1gFdSQznu05cU+D4HroQ_eEmN393f6oHKUTR9VmA@mail.gmail.com" class="">
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<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">And there's not a mention in the docos that this is even a thing. I feel like dragging whoever's responsible to the headmaster's office.</font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">Ok that seems to work -- in the sense that pattern-matching on it yields an `x` that's unusable on RHS just as much as the `Any` decl with implicit `forall`, or with no type signature at all.</font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">What would be useful is to be able to introduce a constraint into the sig, so I can do something like</font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">> foo (Any2 x) y = x == y</font></pre>
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<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">After playing with it, all I'm getting is weird rejections. </font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">> pattern Any2 :: forall . forall a. () => (Eq a) => a -> Some</font></pre>
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</blockquote><p class=""><font face="arial, sans-serif" class="">Your first error message seems
like a case of bad error messages. <br class="">
Just out of curiosity, I've tried writing the following: <br class="">
</font></p><p class=""><font face="arial, sans-serif" class="">pattern Refl :: forall a. forall .
() => (Eq a) => Bool -> a<br class="">
pattern Refl a <- (refl -> a)<br class="">
<br class="">
refl :: Eq a => a -> Bool <br class="">
refl a = a == a</font></p><p class=""><font face="arial, sans-serif" class="">This should not be accepted.
Indeed I get the error message with "fewer arrows", although it
seems like the given type signature has exactly the number of
arrows it needs. The problem here is the misplaced constraint. <br class="">
GHC checks the arity of `Eq a => Bool -> a`, which it
argues has zero arrows (->) at the top of the type. <br class="">
Note that the correct type signature would be <br class="">
</font></p><p class=""><font face="arial, sans-serif" class="">pattern Refl :: forall a. (Eq a)
=> forall . () => Bool -> a</font></p>
<blockquote type="cite" cite="mid:CABU_mxinuS1gFdSQznu05cU+D4HroQ_eEmN393f6oHKUTR9VmA@mail.gmail.com" class="">
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</font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">> * Pattern synonym `Any2' has one argument
> but its type signature has 1 fewer arrows
</font></pre>
<pre style="white-space: pre-wrap;" class=""><font face="arial, sans-serif" class="">I need to put the constraints inside the scope of the `forall a.`. A single `(Show a) => a -> Some` complains no instance provided.</font></pre>
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</blockquote><p class=""><font face="arial, sans-serif" class="">The problem here is, that `Some`
does not carry around any information that its argument has an
`Eq` instance. It was not declared with such a constraint. Thus,
pattern matching on `Some` cannot bring any `Eq` instance into
scope. This is what the second error is trying to tell you.
`Some` does not carry around enough information for `Any` to
provide an `Eq` constraint when pattern matched. </font></p><p class=""><font face="arial, sans-serif" class="">
And as a quick note: Even if we do define `Some :: Eq a => a
-> Some` your example still does not work, since x and y are
not guaranteed to be of the same type. But we could write: <br class="">
</font></p><p class=""><font face="arial, sans-serif" class="">data Some where <br class="">
Some :: Eq a => a -> Some <br class="">
</font></p><p class=""><font face="arial, sans-serif" class="">pattern Any2 :: forall . forall a.
(Eq a) => a -> Some<br class="">
pattern Any2 a = Some a<br class="">
</font></p><p class=""><font face="arial, sans-serif" class="">foo :: Some -> Bool <br class="">
foo (Any2 x) = x == x</font></p>
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