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copied!<p>My experience is that sugaring every term intuitively seems unattractive, but is worth pursuing.</p> <p>An interest in persistent storage has led me by a circuitous route to consider the problems of mixing expression and atomic values. To support this, I decided to separate them completely in the type system; thus Int, Char etc. are the type constructors only for integer and character values. Expression types are formed with the polymorphic type constructor Exp; e.g. Exp Int refers to a value which reduces in one step to Int.</p> <p>The relevance of this to your question arises when we consider evaluation. At an underlying level, there are primitives which require atomic values: COND, addInt etc. Some people refer to this as forcing an expression, I prefer to see it simply as a cast between values of different types.</p> <p>The challenge is to see if this can be done without requiring explicit reduction directives. One solution is exactly as you suggest: i.e. to treat coercion as a form of overloading.</p> <p>Say we have an input script: <code>foo x</code></p> <p>Then, after sugaring, this becomes: <code>(coerce foo) (coerce x)</code> </p> <p>Where, informally:</p> <pre><code>coerce :: a -> b coerce x = REDUCE (cast x) if a and b are incompatible x otherwise </code></pre> <p>Thus <em>coerce</em> is either identity or an application of <em>cast</em> where <em>b</em> is the return type for a given context.</p> <p><em>cast</em> can now be treated as a type class method, e.g.</p> <pre><code>class Cast a, b where {cast :: a -> b }; -- ¬:: is an operator, literally meaning: don’t cast --(!) is the reduction operator. Perform one stage of reduction. -- Reduce on demand instance Cast Exp c, c where { inline cast = ¬::(!)(\x::(Exp c) -> ¬::(!)x) }; </code></pre> <p>The <code>¬::</code> annotations are used to suppress the coerce syntactic sugaring.</p> <p>The intention is that other instances of <code>Cast</code> can be introduced to extend the range of conversions, although I haven't explored this. As you say, overlapping instances seem necessary.</p>

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