Calculating the length of an array in haskell

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It works when loaded from file, but not when typed into ghci. Why? 2 answers

---

What you have is two declarations, the second of which shadows the first.

You need to declare len as one function with two clauses. In GHCi, you can do that like this:

:{
let len [] = 0
    len (h:t) = 1 + len t
:}

the :{ ... :} form lets you enter multi-line declarations as you would in a *.hs file.

GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.

Prelude> let len [] = 0
Prelude> let len (h:t) = 1 + len t -- this shadows the earlier len
Prelude> len [1, 2, 3]
*** Exception: <interactive>:3:5-25: Non-exhaustive patterns in function len 
    -- exception because the new len doesn't handle an empty list

Prelude> :{
Prelude| let len [] = 0
Prelude|     len (h:t) = 1 + len t
Prelude| :}
Prelude> len [1, 2, 3]
3
Prelude>

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