CODEWITHSUNDEEP
haskellghcstandard-library
TheIronKnuckle
TheIronKnuckle

Reputation: 7294

Going through the source code for the prelude brings up weirdness

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?

Upvotes: 15

Views: 516

Answers (2)

Daniel Fischer
Daniel Fischer

Reputation: 183868

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.

Upvotes: 21

Daniel Wagner
Daniel Wagner

Reputation: 152707

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.

Upvotes: 9

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