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POHigher order types in OCaml (phantom types subtyping)
 

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  1. POHigher order types in OCaml (phantom types subtyping)
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    <p>I'm using phantom types to emulate the state of a stack, as a wrapper module for ocaml-lua (Lua communicates with C/OCaml through a stack). Small code example:</p> <pre><code>type 's t type empty type 's table type top val newstate : unit -&gt; empty t (* stack empty *) val getglobal : empty t -&gt; top table t (* stack: -1 =&gt; table *) </code></pre> <p>Certain stack operations are possible on both tables and array-tables (no real arrays in Lua); some aren't. So if I have types</p> <pre><code>type 's table type 's array </code></pre> <p>I want a kind like table-or-array for functions that operate on both types, but still being able to forbid e.g. <code>rawgeti</code> (array operation) on tables. <code>objlen</code> is a stack operation that returns the "length" of the element at the top of the stack. This element can be both table or array-table. At the moment, the wrapper function is defined as follows:</p> <pre><code>val objlen : (top table) t -&gt; int </code></pre> <p>What I want is </p> <pre><code>val objlen : (top table-or-array) t -&gt; int </code></pre> <p>That is, <code>array</code> and <code>table</code> is subtypes of <code>table-or-array</code>.</p> <p>Any ideas?</p> <p>Regards Olle</p> <p><strong>Edit</strong></p> <p>After considerations, I came up with this:</p> <pre><code>module M : sig type ('s, 't) t (* New Lua state with empty stack *) val newstate : unit -&gt; (unit, unit) t (* Get table *) val getglobal : ('a) t -&gt; ([&lt; `table | `string | `number | `fn], 'a) t (* Get array index and put "anything" on top of stack *) val rawgeti : ([`table], 'a) t -&gt; ([&lt; `table | `string | `number | `fn], [`table] * 'a) t (* String on top of stack *) val tostring : ([`string], _) t -&gt; string (* Table or array-table on top of stack *) val objlen : ([`table], _) t -&gt; int val pop : ('a, 'b * 'c) t -&gt; ('b, 'c) t end = struct type top type ('s, 't) t = string (* Should really be Lua_api.Lua.state *) (* Dummy implementations *) let newstate () = "state" let gettable s = s let getarray s = s let rawgeti s = s let tostring s = "Hello phantom world!" let objlen s = 10 let pop s = s end </code></pre> <p>The stack at type level should now know neither more nor less than the stack itself. E.g. rawgeti will push on stack any type.</p>
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    1. USOlle Härstedt
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    1. COWhat about `type ('s, 't) t val objlen : (top, [< \`table | \`array]) t -> int`?
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    2. COActually the type `[< \`table | \`array]` is the same as `[\`table | \`array]`.
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