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Binding and evaluation order in module/type/value languages
- Dawid Toton
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Date: | 2010-05-17 (19:21) |
From: | Dawid Toton <d0@w...> |
Subject: | Binding and evaluation order in module/type/value languages |
Thinking about the discussion in the recent thread "Phantom types" [1], I have created the following piece of code that aims to demonstrate binding and evaluation order that takes effect in all three levels of OCaml. My question is: what are the precise rules is the case of type language? I have impression that it is lazy and memoized evaluation. But this my guess looks suspicious. I don't intend this question to be about inner working of the compiler, but about the definition at the conceptual level. (* 1. Module language; side effect = create fresh record type; test = type equality test *) module type T = sig type t end module R (T:T) = struct type r = {lab : int} end module TF = struct type t = float end module TS = struct type t = string end module R1 = R(TF) module R2 = R(TF) module R3 = R(TS) let test12 (k : R1.r) (l : R2.r) = (k=l) (* pass => R1.r = R2.r *) let test13 (k : R1.r) (l : R3.r) = (k=l) (* pass => R1.r = R3.r *) (* Conclusion: RHS evaluated at the mapping definition point *) (* 2. Type language; side effect = create fresh record type; test = type equality test *) type 't r = {lab : int} type tf = float type ts = string type r1 = tf r type r2 = tf r type r3 = ts r let test12 (k : r1) (l : r2) = (k=l) (* pass => r1 = r2 *) let test13 (k : r1) (l : r3) = (k=l) (* fail => r1 ≠r3 *) (* Conclusion: RHS evaluated some time after the mapping is applied; sort of memoization at the conceptual level *) (* 3. Value language; side effect = create fresh int; test = value equality test *) let r t = Oo.id (object end) let tf = 0. let ts = "A" let r1 = r tf let r2 = r tf let r3 = r ts let test12 = assert (r1 = r2) (* fail => r1 ≠r2 *) let test13 = assert (r1 = r3) (* fail => r1 ≠r3 *) (* Conclusion: RHS evaluated exactly at the point of mapping application *) Dawid [1] http://groups.google.com/group/fa.caml/browse_thread/thread/0df560ee78e0f75f#