[Haskell-cafe] 1 = 0?

[J. Waldmann]

> Interesting. I just told GHC "let 5 = 3" and got no complaint. 

lazy pattern matching semantics, see:

ghci -XBangPatterns

Prelude> let {  0 = 1 } in 0
0

Prelude> let { ! 0 = 1 } in 0
*** Exception: <interactive>:3:7-13: Non-exhaustive patterns in pattern binding