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Haskell endless loop with just repetitive action

I have code like this:

Prelude> let n = [1,2,3,4]
Prelude> n
[1,2,3,4]
Prelude> 0:n
[0,1,2,3,4]
Prelude> let n = 0:n

And when I type the Haskell interpreter after the top level:

Prelude> n

I am getting infinite result: 

[0,0,0,0,0,0,0,0,0

And where the seal "0" is endless.

Why am I getting this result?
Is there some kind of recursive stuff and why / how does it work at the interpreter level?
Can I catch a stack overflow, what's on GHCi or not?

Thanks,
Best wishes!

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3 answers


What Josh says is that you define n as:



0:n.  -- note n still equals 0:n, just like you said
0:0:n. -- note n _still_ equals 0:n
0:0:0:n
...
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The new binding n

shades the old one. You don't reassign variables in Haskell.



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Haskell let

is similar to letrec

other languages ​​like ML: bindings are allowed to be recursive. In other words, it n = 0:n

defines n

as a list, where the first element 0

is and the rest of the list is equal n

. This means the second element n

is equal to the first element n

, which is 0

, etc.

In Haskell, because of laziness, infinite lists are fine. If you only use the first 10 elements of the list, then the 11th element and subsequent ones will never be evaluated.

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