You can identify a Solidity source file by the .sol extension. It has various versions, as programming languages usually do. The latest version at the time of writing this book is 0.4.17.
In the source file, you can use the pragma Solidity directive to mention the compiler version for which the code is written. For example:
pragma Solidity ^0.4.17;
It is important to note that the source file will not compile with compiler versions earlier than 0.4.17 and later than 0.5.0 (this second condition is added using ^). Compiler versions between 0.4.17 and 0.5.0 are most likely to include bug fixes and less likely to break anything.
We can specify more complex rules for the compiler version; the expression follows those used by npm.
