# [Haskell-cafe] Guards (Was: Some random newbie questions)

Lemming schlepptop at henning-thielemann.de
Sat Jan 8 12:46:04 EST 2005

```Jon Cast wrote:

> do notation, and Haskell lacks a direct analogy to Lisp's cond.
>
> case () of
>   () | p1 -> e1
>      | p2 -> e2
>      ...

No problem:

select :: a -> [(Bool, a)] -> a
select def = maybe def snd . List.find fst

Use it this way:

select defaultE
[(p1, e1),
(p2, e2)]

Would be a nice Prelude function.

>  parseCmd ln
>    | Left err <- parse cmd "Commands" ln
>      = BadCmd \$ unwords \$ lines \$ show err
>    | Right x <- parse cmd "Commands" ln
>      = x
>
>
>  parseCmd ln = case parse cmd "Commands" ln of
>    Left err -> BadCmd \$ unwords \$ lines \$ show err
>    Right x  -> x

Really, the second alternative is cleaner in my opinion.

> Furthermore, guards are an extension of pattern matching, which means
> you can write code like this:
>
>  xn !! n     | n < 0  = error "Prelude.(!!): Negative index"
>  [] !! n              = error "Prelude.(!!): Index overflow"
>  (x:xn) !! n | n == 0 = x
>  (x:xn) !! n          = xn !! (n - 1)
>
> Exactly one equation for each edge in the control-flow graph, which is
> nice and not easily done (I'm not sure it's even possible) without
> guards.

At least one guard can nicely be avoided:

(x:xn) !! n  =  if n == 0 then x else xn !! (n - 1)

But I see that guards can be used to let pattern matching fail.

> Pattern guards are also nice for implementing â€˜viewsâ€™:
>
>  -- | Convert an 'XMLData' into an equivalent application of
>  -- 'Balanced', if possible.  In any case, return an equivalent data
>  -- structure.
>  balance (Balanced es) = Balanced es
>  balance (LeftLeaning (LeftBalanced e:es))
>    | Balanced es' <- balance (LeftLeaning es)
>    = Balanced (e:es')

I don't know what this means exactly, but I think I can transform it
formally to:

balance e'@(LeftLeaning (LeftBalanced e:es)) =
case balance (LeftLeaning es) of
Balanced es' -> Balanced (e:es')
_ -> e'

This way it is more clear for me, that 'balance' can return something
different from 'Balanced' and that the data is returned unchanged in
this case.

>  balance (LeftLeaning []) = Balanced []
>  balance (RightLeaning [("", "", es)]) = Balanced es
>  balance (RightLeaning []) = Balanced []
>  balance e = e

> Well, I could never do without them.

Sometimes I see people abusing guards, e.g. they write a 'length x == 1'
guard, where the pattern '[x0]' would be clearly the better choice. So
I'm always thinking twice before using a guard.
```