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    <p>Doesn't tacit programming correspond pretty closely to combinator logic or <strike>pointless</strike> point-free style in Haskell? For instance, while I don't know J from what I gather a "fork" translates three functions <code>f</code>, <code>g</code>, and <code>h</code> and an argument <code>x</code> into an expression <code>g (f x) (h x)</code>. The operation of "apply multiple functions to a single argument, then apply the results to each other in sequence" is a generalization of <strike>Curry's</strike> Schönfinkel's <strong>S</strong> combinator and in Haskell corresponds to the <code>Applicative</code> instance of the Reader monad.</p> <p>A <code>fork</code> combinator in Haskell such that <code>fork f g h x</code> matches the result specified above would have the type <code>(t -&gt; a) -&gt; (a -&gt; b -&gt; c) -&gt; (t -&gt; b) -&gt; t -&gt; c</code>. Interpreting this as using the Reader functor <code>((-&gt;) t)</code> and rewriting it for an arbitrary functor, the type becomes <code>f a -&gt; (a -&gt; b -&gt; c) -&gt; f b -&gt; f c</code>. Swapping the first two arguments gives us <code>(a -&gt; b -&gt; c) -&gt; f a -&gt; f b -&gt; f c</code>, which is the type of <code>liftA2</code>/<code>liftM2</code>.</p> <p>So for the common example of computing the average, the fork <code>+/ % #</code> can be translated directly as <code>flip liftA2 sum (/) (fromIntegral . length)</code> or, if one prefers the infix <code>Applicative</code> combinators, as <code>(/) &lt;$&gt; sum &lt;*&gt; fromIntegral . length</code>.</p> <blockquote> <p>If not, is there a technical issue that makes this impossible, or is it just not worth doing?</p> </blockquote> <p>In Haskell at least, I think the main issue is that extremely point-free style is considered obfuscated and unreadable, particularly when using the Reader monad to split arguments.</p>
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    1. POHave J style adverbs, forks etc been emulated via libraries in mainstream functional languages?
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    1. COAs a slightly depressing aside, this answer, in which the second sentence says "...while I don't know J...", has given me the second-highest number of upvotes for answers in the `j` tag. SO needs more J programmers!
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    2. COIn addition this question now has the most upvotes in the `J` tag. And, I've never actually written and executed a `J` program. I guess I know about it though - in fact I stayed at Ken Inverson's place in Toronto for a weekend in 1995 (my dad was visiting to Ken work on `J` stuff, and I was doing my PhD in CMU - yeah, I guess I'm second generation PLer).
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    3. COBTW - nice answer. I was thinking along similar lines, but you've put it better. One difference is that in `J` the fork is syntactically implicit, as are hooks. I doubt that could be emulated, and personally implicit forks and hooks seem like they are going to far to me. The other difference is the implementation which sclv covered - and J actually does handle large (and even huge) arrays quite efficiently.
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