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copied!<p>For languages that support currying and partial-application easily, there is one compelling series of arguments, originally from Chris Okasaki:</p> <ul> <li><strong>Put the data structure as the last argument</strong></li> </ul> <p>Why? You can then <a href="http://www.haskell.org/pipermail/libraries/2005-August/004297.html" rel="noreferrer">compose operations on the data</a> nicely. E.g. <code>insert 1 $ insert 2 $ insert 3 $ s</code>. This also helps for <a href="http://www.haskell.org/pipermail/libraries/2005-August/004323.html" rel="noreferrer">functions on state</a>.</p> <p>Standard libraries such as "containers" <a href="http://hackage.haskell.org/packages/archive/containers/0.4.0.0/doc/html/Data-Map.html#g:5" rel="noreferrer">follow this convention</a>.</p> <p>Alternate arguments are sometimes given to put the data structure first, so it can be closed over, yielding functions on a static structure (e.g. lookup) that are a bit more concise. However, the broad consensus seems to be that this is less of a win, especially since it pushes you towards heavily parenthesized code.</p> <ul> <li><strong>Put the most varying argument last</strong></li> </ul> <p>For recursive functions, it is common to put the argument that varies the most (e.g. an accumulator) as the last argument, while the argument that varies the least (e.g. a function argument) at the start. This composes well with the data structure last style.</p> <hr> <p>A summary of the Okasaki view is given in <a href="http://hackage.haskell.org/packages/archive/EdisonAPI/1.2.1/doc/html/Data-Edison.html" rel="noreferrer">his Edison library</a> (again, another data structure library):</p> <ul> <li><strong>Partial application</strong>: arguments more likely to be static usually appear before other arguments in order to facilitate partial application.</li> <li><strong>Collection appears last</strong>: in all cases where an operation queries a single collection or modifies an existing collection, the collection argument will appear last. This is something of a de facto standard for Haskell datastructure libraries and lends a degree of consistency to the API.</li> <li><strong>Most usual order</strong>: where an operation represents a well-known mathematical function on more than one datastructure, the arguments are chosen to match the most usual argument order for the function.</li> </ul>

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