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PO
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Id
4244620
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0
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0
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0
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CreationDate
2010-11-22T10:49:46.097
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0
LastActivityDate
2015-09-08T10:27:27.147
LastEditDate
2015-09-08T10:27:27.147
LastEditorUserId
340297
OwnerUserId
298143
ParentId
4242677
PostTypeId
2
Score
23
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0
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text
Body
<code>_</code> denotes a weakly polymorphic variable : it is in a position where it cannot be generalized. There are two explanations related to weak polymorphism in the OCaml FAQ: see <a href="http://caml.inria.fr/resources/doc/faq/core.en.html#eta-expansion" rel="noreferrer">A function obtained through partial application is not polymorphic enough</a> and the next one. This generally happens when you're using a non-local reference (whose type cannot be generalized), or when defining polymorphic functions that are not syntactically functions (they do not begin with <code>fun x -> ..</code> but rather a function application). In some cases there is an easy fix (eta-expansion, see the FAQ), sometimes there isn't, and sometimes your program was just unsound. An easy example : <code>let a = ref []</code> does not get the polymorphic <code>a list ref</code> type. Otherwise you could use both as a <code>int list</code> and a <code>bool list</code>, and mix elements of different types by mutating the reference. It instead get a <code>'_a list ref</code> type. This mean that the type is not polymorphic, but merely unknown. Once you do something with <code>a</code> with a particular type, it fixes <code>'_a</code> once and for all. <pre><code># let a = ref [];; val a : '_a list ref = {contents = []} # let sum_of_a = List.fold_left (+) 0 !a;; val sum_of_a : int = 0 # a;; - : int list ref = {contents = []} </code></pre> For an in-depth explanation of value restriction and the "relaxed" value restriction actually implemented in the OCaml type checker, see the <a href="http://caml.inria.fr/pub/papers/garrigue-value_restriction-fiwflp04.pdf" rel="noreferrer">Relaxing the Value Restriction</a> paper by Jacques Garrigue (2004).
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1. POWhat is the difference between 'a and '_l?
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1. PTAnswer
UserLastEditorUserId
1. USCatalin Hritcu
UserOwnerUserId
1. USgasche
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1. POWhat is the difference between 'a and '_l?
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