Implicit parameters and conversions are powerful tools in Scala increasingly used to develop concise, versatile tools such as DSLs, APIs, libraries\u2026<\/p>\n
When used correctly,\u00a0they reduce the verbosity of Scala programs thus providing\u00a0easy to read code. But their most important feature is that they offer\u00a0a way to make your libraries functionality extendible without having to change their code nor needing\u00a0to distribute\u00a0it.<\/p>\n
![\"\"](\"http:\/\/static.giantbomb.com\/uploads\/original\/1\/18723\/1839615-dont_panic_thumb.jpg\")
A great power comes with a great responsibility however. For new comers, as well as for relatively experienced Scala users, they can become a galaxy of confusions and pitfalls derived from the fact that the use of implicit values imply the compiler making decisions not obviously described in the code and following a set of rules with some unexpected results.<\/p>\n
This post pretends to shed some light on the use of implicit values. Its\u00a0content isn\u2019t 100% original, it is just a tourist guide through this full of marvels, and sometimes dangerous, code jungle.
\nAs most of those monstrous things that make us shiver, implicit values are mostly harmless once you get to know them.<\/p>\n
In essence, implicit values are no more than a tool for\u00a0providing the compiler with a way to continue when the code it is\u00a0compiling lacks some elements which should had been explicitly specified. These elements can only be:<\/p>\n
And are fulfilled with those implicit values present in the context of the incomplete code. The straight line is the shortest path from one point to another, similarly the shortest path from a type to another is a function. What is then the most obvious Scala language tool to describe conversions from a type to another?<\/p>\n Yes, functions! A conversion is just a function from T to S<\/p>\n Ok, but just defining a function doesn\u2019t imply that all places where S is expected will be also be able to receive a parameter of type T. Let\u2019s solve that:<\/p>\n Making the definition as\u00a0implicit\u00a0<\/strong>tells the compiler that it should try to apply\u00a0conversion\u00a0<\/em>when S is expected but T is passed.<\/p>\n The provided type is known as the conversion\u00a0source\u00a0<\/strong>and the type the conversion provides is the\u00a0target<\/strong>.<\/p>\n It is obvious that trying something like:<\/p>\n Won\u2019t even compile because a type mismatch. However, once an implicit conversion from A (source<\/strong>) to B (target<\/strong>) is available (in context; Don\u2019t worry, this concept will be explained soon), the compiler will try to apply it in order to fix the type error:<\/p>\n Finally, A elements become also valid actual parameters for formal parameters of type B:<\/p>\n
\nTherefore,\u00a0there\u00a0is one type of implicit value for each kind of element above:\u00a0Implicit conversions<\/strong>\u00a0and\u00a0implicit parameters<\/strong>.<\/p>\n<\/h2>\n
The goal of implicit conversions<\/h2>\n
def\u00a0conversion(x:\u00a0T):\u00a0S\u00a0=\u00a0???<\/pre>\n
\nimplicit\u00a0def\u00a0conversion(x:\u00a0T):\u00a0S\u00a0=\u00a0???<\/pre>\n<\/div>\n
\u00a0\ncase\u00a0class\u00a0A(x:\u00a0Int)\ncase\u00a0class\u00a0B(m:\u00a0String, v:\u00a0Double) {\n\u00a0def\u00a0method\u00a0=\u00a0println(s\"hello world $m $v\")\n}\n\u00a0\nval\u00a0a\u00a0=\u00a0A(10)\nvar\u00a0b:\u00a0B\u00a0=\u00a0B(\"what is the meaning of life?\",\u00a042.0)<\/pre>\nb = a<\/pre>\n
implicit def conv(v: A): B = B(\"Some String\", v.x.toDouble)\n\u00a0\nb = a\u00a0\/\/Now, this is equivalent to write\nb = conv(a)\u00a0\/\/ so it works! (A is the source type and B the target)<\/pre>\n
\n\u00a0This also applies to attribute accesses as well as method calls:<\/div>\n<\/div>\nDouble dval\u00a0=\u00a0a.v\u00a0\/\/ A doesn't have a v attribute, but B does...\n\/\/ so it is translated to: Double dval = conv(a).v\nString m\u00a0=\u00a0a.m\u00a0\/\/ m = conv(a).m\na.method\u00a0\/\/ conv(a).method<\/pre>\n
def\u00a0f(x:\u00a0B):\u00a0Double\u00a0=\u00a0x.m.size * x.v\nf(a)\u00a0==\u00a0f(conv(a))<\/pre>\n
![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/coins.png\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/etta-james-copy-150x150.jpg\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/freeman-dent2-1024x684.jpg\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/implicit_param.png\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/implicit_convs_target.png\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/implicit_conv_source.png\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/implicit_convs_map.png\")
<\/p>\n![\"\"](\"http:\/\/blog.stratio.com\/wp-content\/uploads\/2016\/05\/fordperfect-300x214.png\")
<\/p>\n