[Haskell-cafe] Word rigid in "`a' is a rigid type variable..."

Hi Cafe, in an example function f :: a -> Bool f a = let b = "x" in a == b compiler complains with `a' is a rigid type variable bound by the type signature for f :: a -> Bool I'm puzzled with the choice of word 'rigid' here. I see these types as - 'b' has "rigid/unchangeable" type (only String), and - 'a' has "soft/variable" type (any type, no constraints). Why is it called rigid? Where does the meaning (in this context) come from? Best regards, vlatko