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copied!<p>Jack's answer is correct. I'll add a bit more context. <code>(%%)</code> is the untyped splice operator (that is, it splices a <code>Quotations.Expr</code> into another typed or untyped quotation), while <code>(%)</code> is the typed splice operator (that is, it splices a <code>Quotations.Expr<'t></code> for some <code>'t</code> into another typed or untyped quotation). <code>(a : ty)</code> is just a type annotation, so <code>(%%row : float[])</code> indicates that when <code>row</code> is spliced into the quotation the result is a <code>float[]</code>. Without this annotation, <code>%%row</code> could be a value of any type, and the compiler would be unable to infer what we mean by the <code>.[]</code> indexer (just as it can't infer the type of <code>arr</code> in <code>fun arr i -> arr.[i]</code>).</p> <p>In case it's helpful, here are some alternative ways to express roughly the same thing as <code><@@ (%%row:float[]).[i] @@></code>:</p> <ul> <li><p>We can convert the untyped quotation to a typed quotation before splicing:</p> <pre><code>let typedRow = Quotations.Expr.Cast<float[]> row <@@ %typedRow.[i] @@> </code></pre> <p>Here we are using the typed splice operator <code>(%)</code>, so the compiler knows that <code>%typedRow</code> is a <code>float[]</code> and that the <code>.[]</code> operator is applicable.</p></li> <li><p>We can use a different way of indexing into the array so that F#'s type inference can determine the type of <code>%%row</code> without an annotation:</p> <pre><code><@@ Array.get %%row i : float @@> </code></pre> <p>Here, the <code>Array.get</code> method takes an <code>'a[]</code> as an argument and we add a type annotation which indicates that the result is a float, so F# will infer that <code>%%row</code> is a <code>float[]</code>.</p></li> </ul>

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