问题:

I was looking for the definition of seq and came across this weirdness. Why do all these functions have the same/similar definitions?

seq :: a -> b -> b
seq = let x = x in x

inline :: a -> a
inline = let x = x in x    

lazy :: a -> a
lazy = let x = x in x

There are many more with this definition in the source code. What's going on?

回答1:

What's going on is that these functions cannot be implemented in Haskell, but they should appear in the docs. Since haddock needs a syntactically correct (and well-typed) definition for each signature, the source must contain dummy definitions. Further, at the point where they are defined (in the ghc-prim package), error (and hence undefined) are not yet available, so the more obvious seq = error "Not implementable in Haskell" can't be used, thus the circular definition.

回答2:

These definitions are a ruse: they're provided primitively by the GHC runtime. It turns out that the infinite loop let x = x in x can be given any type, so it's as good a ruse definition as any.