I've got this code:
oof [] = 1
oof [(i,j)] = i+j
oof [(i,7),(5,j)] = i*j
oof (_:_:xs) = foo xs
and I typed it into ghci. When I try to execute the expression:
oof [(3,7),(5,2)]
it should make use of the third line and return 6, but I'll get the error message:
Exception: <interactive>:11:1-21: Non-exhaustive patterns in function foo
Why do I get this exception?
You likely typed it one line at a time. You should wrap multi-line statements between :{ and :}, so:
ghci> :{
ghci| oof [] = 1
ghci| oof [(i,j)] = i+j
ghci| oof [(i,7),(5,j)] = i*j
ghci| oof (_:_:xs) = oof xs
ghci| :}
ghci> oof [(3,7),(5,2)]
6If you enter these line-by-line, you each time define a new more locally scoped function oof, that only works with the last defined clause.