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Actually I was thinking about how to determine function purity in a static analysis manner for a good while, spurred on by how the Chrome developer console some

Actually I was thinking about how to determine function purity in a static analysis manner for a good while, spurred on by how the Chrome developer console sometimes gives "lazy previews" of typed Javascript statements, but only ones that have no side effects. As it turns out V8 has a feature which can guess if an expression is pure or impure, erring on the side of impure due to undecidability and other random issues. It does this by whitelisting a set of opcodes/built-ins that can potentially mutate state or not and running the statement in a separate VM instance with a debug hook set to throw an exception if an impure opcode/built-in is called. The parent developer tools code then prints the expression if the VM instance returns successfully. https://chromium.googlesource.com/v8/v8.git/+/ba5bac8cebe91c585024c67687ced8fe1baed833/src/debug/debug-evaluate.cc#267 https://chromium.googlesource.com/v8/v8.git/+/ba5bac8cebe91c... I wanted something similar in LuaJIT but it's pretty non-trivial to accomplish. I guess it's one of those mildly useful innovations you can only have with a lot of developer time.

Animats 6y ago
Checking for function purity is easier in some languages than others. It's mostly a transitive closure thing. Pure functions can call only pure functions. It's tough in Javascript, because function declaration is dynamic and you can rebind. It's not that tough in a more static language, where you can tell who calls whom at compile time. Way too many years ago, I did that.[1], starting at line 204. Knowing that a function is pure has many uses. You can optimize, using that x=y => f(x)=f(y). Pure functions can safely be used in assertions and conditional debug code. [1] https://github.com/John-Nagle/pasv/blob/master/src/CPC2/p2trutil.i https://github.com/John-Nagle/pasv/blob/master/src/CPC2/p2tr...

Animats 6y ago

Checking for function purity is easier in some languages than others. It's mostly a transitive closure thing. Pure functions can call only pure functions. It's tough in Javascript, because function declaration is dynamic and you can rebind. It's not that tough in a more static language, where you can tell who calls whom at compile time. Way too many years ago, I did that.[1], starting at line 204. Knowing that a function is pure has many uses. You can optimize, using that x=y => f(x)=f(y). Pure functions can safely be used in assertions and conditional debug code. [1] https://github.com/John-Nagle/pasv/blob/master/src/CPC2/p2trutil.i https://github.com/John-Nagle/pasv/blob/master/src/CPC2/p2tr...