types - Haskell - fmap fmap doesn't work -

-

i'm using ghci (version 6.12.3) play bit haskell. read functors , applicative functors thought if couldn't similar <*> of applicative functors implemented using functor's primitives. after thinking came fmap fmap have (nearly) ideal type of

functor f => f (a -> b) -> f (f -> f b) or more generically

(functor f1, functor f2) => f1 (a -> b) -> f1 (f2 -> f2 b)

i tried

let q = fmap fmap

i got following error

<interactive>:1:8:     ambiguous type variable `f1' in constraint:       `functor f1' arising use of `fmap' @ <interactive>:1:8-16     probable fix: add type signature fixes these type variable(s)  <interactive>:1:13:     ambiguous type variable `f' in constraint:       `functor f' arising use of `fmap' @ <interactive>:1:13-16     probable fix: add type signature fixes these type variable(s)

writing above type signature suggested didn't help. craziest thing when typed :t fmap fmap got equivalent type above.

what doing wrong? why fmap fmap give type error although ghci finds type it?

looks you're running monomorphism restriction.

trying example in ghci -xnomonomorphismrestriction gives expected result.

you can subvert writing let f x = fmap fmap $ x. monomorphism restriction applies top-level definitions "look like" values, i.e. f = something, introducing explicit argument causes not apply anymore. not apply if not @ top level (for example in where clause). more details, see link.


Comments

Post a Comment

Popular posts from this blog

objective c - Change font of selected text in UITextView -

-
can suggest me how change font of selected text in uitextview.i cannot use uiwebview since have edit text. have seen pages app.how possible in app. see formatting text within uilabel differently . since need editing, should @ javascript-based editors such ckeditor or tinymce . highly recommend wwdc 2011 video session 511: "rich text editing in safari on ios."
Read more

php - Accessing POST data in Facebook cavas app -

-
so i've got simple form in canvas / iframe facebook app, , i'm trying pass along values post. reading on s.o. , fb's latest docs, understand it, data send via post form can accessed on receiving end $_request object. i read on thread on s.o. in order post forms work need pass along input named "signed_request" value current signed_request (i have signed request working ok otherwise...all login , authentication stuff working fine). isnt mentioned anywhere in official fb docs. so problem comes in $_request object signed request, , bunch of other session stuff. form inputs found. the way can read them set method of form "request" isn't real form method. takes of inputs , sends them args in url. horrible. here's sample page canvas app form i'm using try debug (leaving out authentication stuff): <form enctype="application/x-www-form-urlencoded" method="post" target="_top" id="my_form" action=...
Read more

c# - Getting control value when switching a view as part of a multiview -

-
i have following code in aspx page: <asp:button id="display_button" runat="server" text="display" onclick="button1_click" /> &nbsp; <asp:button id="edit_button" runat="server" text="edit" onclick="button2_click" /> &nbsp; <asp:button id="save_button" runat="server" text="save" onclick="button3_click" visible="false" /> &nbsp; <asp:multiview id="multiview1" runat="server" activeviewindex="0"> <asp:view id="view1" runat="server"> <asp:formview id="view_program" runat="server"> <itemtemplate> <%# eval("status").tostring().trim() %> </itemtemplate> </asp:formview> </asp:view> <asp:view id="view2" runat="server"> ...
Read more