[Haskell-cafe] Laziness question

michael rice nowgate at yahoo.com
Sat Jul 31 11:56:04 EDT 2010

From: http://en.wikibooks.org/wiki/Haskell/Laziness

Given two functions of one parameter, f and g, we say f is stricter than g if f x evaluates x to a deeper level than g x


   1. Which is the stricter function?

f x = length [head x]
g x = length (tail x)

Prelude> let f x = length [head x]
Prelude> let g x = length (tail x)
Prelude> f undefined
Prelude> g undefined
*** Exception: Prelude.undefined

So, g is stricter than f?

Wouldn't both functions need to evaluate x to the same level, *thunk* : *thunk* to insure listhood?

f x = length [head *thunk* : *thunk*]
g x = length (tail *thunk* : *thunk*)


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