The idris2 has the following example for FFI, which adds two integers in C:
#include <stdio.h>
int add(int x, int y) {
return x+y;
}
(small.c)
I followed the steps, and after linking:
gcc -shared smallc.c -o libsmall.so
and indeed the foreign function add can be called as shown in the example:
%foreign "C:add,libsmall"
add : Int -> Int -> Int
main : IO ()
main = printLn (add 70 24)
:exec main gives 94.
However, if I just evaluate this foreign function as a pure function (add 70 24), then the idris2 REPL does not compute anything, and just echoes back the expression:
add 70 24
add 70 24
So, the pure foreign function only computes inside the IO() valued function when :exec. This is not consistent with what the same documentation claims:
It is up to the programmer to declare which functions are pure, and which have side effects, via PrimIO
In contrast, (+) 70 24 evaluates correctly in the REPL.
(+) 70 24
94
(The above is tested with idris2 0.6.0 under Ubuntu 22.04 LTS.)
My questions are:
How to fix the above example so that the foreign function evaluates as a stand-alone expression?
Why did the builtin pure functions such as (+) work? Is it because they use RefC or some different methods?
Related:
How to write a pure String to String function in Haskell FFI to C++
