main = do
[1 of 1] Compiling Main ( prog.hs, prog.o )
prog.hs:2:26:
No instance for (Num (IO t))
arising from the literal `2' at prog.hs:2:26
Possible fix: add an instance declaration for (Num (IO t))
In the first argument of `(*)', namely `2'
In the first argument of `(+)', namely `2 * a'
In the expression: 2 * a + b
Standard output is empty