Search code examples
haskelliofunctor

Why is getArgs evaluated after fmap method argument?

Why does getArgs gets evaluated after the method argument of fmap?

main::IO()
main=do
    fpath<-fmap head getArgs
    putStrLn fpath

I get the error : Exception: Prelude.head: empty list It seems it applies head on something that has not been computed yet.

I first assumed that it might be another rule about lazyness that i am not aware of being new to Haskell so i tried with :
a<-fmap head getLine # no problem
a<-fmap head (readFile [filename]) # again no problem

So why is getArgs special that gets evaluated after ?

Solution

  • If head breaks on empty list, that means that getArgs did already evaluate, because the value [] was produced and matched by head.

    Most probably you ran your program from ghci, which can produce such an effect. Since head is unsafe, you should check whether there's at least one argument on the list present.