<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><span style="font-size: 20px;" class="">Hi,</span><div class=""><span style="font-size: 20px;" class="">Thanks for the pointers. So I’ve got</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">data M (n :: Nat) a = M [a] deriving Show<br class=""><br class="">t2 :: M 2 Int<br class="">t2  = M [1,2]<br class=""><br class="">t3 :: M 3 Int<br class="">t3 = M [1,2,3]<br class=""><br class="">fx :: Num a => M n a -> M n a -> M n a<br class="">fx (M xs) (M ys) = M (zipWith (+) xs ys)<br class=""></span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">and having </span></div><div class=""><span class=""><span style="font-size: 20px;" class="">g = fx t2 t3</span><br class=""></span></div><div class=""><br class=""></div><div class=""><span style="font-size: 20px;" class="">won’t compile. Which is what I want.</span></div><div class=""><span style="font-size: 20px;" class="">However…</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">t2</span><span style="font-size: 20px;" class=""> </span><span style="font-size: 20px;" class="">::</span><span style="font-size: 20px;" class=""> </span><span style="font-size: 20px;" class="">M</span><span style="font-size: 20px;" class=""> 2 </span><span style="font-size: 20px;" class="">Int</span><br style="font-size: 20px;" class=""><span style="font-size: 20px;" class="">t2  </span><span style="font-size: 20px;" class="">=</span><span style="font-size: 20px;" class=""> </span><span style="font-size: 20px;" class="">M</span><span style="font-size: 20px;" class=""> [</span><span style="font-size: 20px;" class="">1</span><span style="font-size: 20px;" class="">,</span><span style="font-size: 20px;" class="">2</span><span style="font-size: 20px;" class="">]</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">is ‘hardwired’ to 2 and clearly I could make t2 return  a list of any length. </span></div><div class=""><span style="font-size: 20px;" class="">So what I then tried to look at was a general function that would take a list of Int and create the M type using the length of the supplied list. </span></div><div class=""><span style="font-size: 20px;" class="">In other words if I supply a list, xs, of length n then I wan’t  M n xs</span></div><div class=""><span style="font-size: 20px;" class="">Like this</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class=""><span class="">createIntM xs </span><span class="">=</span><span class=""> (</span><span class="">M</span><span class=""> xs) </span><span class="">::</span><span class=""> </span><span class="">M</span><span class=""> (</span><span class="">length</span><span class=""> </span><span class="">xs</span><span class="">) </span><span class="">Int</span></span></div><div class=""><span style="font-size: 20px;" class=""><span class=""><br class=""></span></span></div><div class=""><span style="font-size: 20px;" class="">which compile and has type</span></div><div class=""><div class=""><span style="font-size: 20px;" class="">λ-> :t createIntM </span></div><div class=""><span style="font-size: 20px;" class="">createIntM :: [Int] -> M (length xs) Int</span></div></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">and all Ms created using </span><span style="font-size: 20px;" class="">createIntM  have the same type irrespective of the length of the supplied list.</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">What’s the type jiggery I need or is this not the right way to go?</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">Thanks</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class="">Mike</span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><span style="font-size: 20px;" class=""><br class=""></span></div><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 14 Mar 2019, at 13:12, Frederic Cogny <<a href="mailto:frederic.cogny@gmail.com" class="">frederic.cogny@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">The (experimental) Static module of hmatrix seems (I've used the packaged but not that module) to do exactly that: <a href="http://hackage.haskell.org/package/hmatrix-0.19.0.0/docs/Numeric-LinearAlgebra-Static.html" class="">http://hackage.haskell.org/package/hmatrix-0.19.0.0/docs/Numeric-LinearAlgebra-Static.html</a><div class=""><br class=""><br class=""><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Thu, Mar 14, 2019, 12:37 PM Francesco Ariis <<a href="mailto:fa-ml@ariis.it" class="">fa-ml@ariis.it</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello Mike,<br class="">
<br class="">
On Thu, Mar 14, 2019 at 11:10:06AM +0000, mike h wrote:<br class="">
> Multiplication of two matrices is only defined when the the number of columns in the first matrix <br class="">
> equals the number of rows in the second matrix. i.e. c1 == r2<br class="">
> <br class="">
> So when writing the multiplication function I can check that  c1 == r2 and do something.<br class="">
> However what I really want to do, if possible, is to have the compiler catch the error. <br class="">
<br class="">
Type-level literals [1] or any kind of similar trickery should help you<br class="">
with having matrices checked at compile-time.<br class="">
<br class="">
[1] <a href="https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/type-level-literals.html" rel="noreferrer" target="_blank" class="">https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/type-level-literals.html</a><br class="">
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</blockquote></div></div>-- <br class=""><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr" class=""><div class=""><font class="">Frederic Cogny<br class=""></font></div><font class=""><font class="">+33 </font><span class="">7 83 12 61 69</span></font><br class=""></div></div>
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