If you have a long if/else if/else chain, you might consider a trivial
case statement with guards. Whether you think this is attractive is a
matter of taste, but it has the fall-through semantics you want and ghc
optimizes the _ pattern matching away:
f x = case () of
_| x == 2 -> 22
_| x == 4 -> 44
_| x == 7 -> 77
_| otherwise -> 55
> f 4
44
> f 9
55
michael rice wrote:
Thanks guys,
I understand what you're telling me, but have some nested IFs and just
want to fall through on one of the ELSES but then I end up with two
ELSES in a row and nothing between them. Oh, well, on to restructuring.
Michael
--- On *Wed, 10/21/09, Tim Wawrzynczak /<[email protected]>/* wrote:
From: Tim Wawrzynczak <[email protected]>
Subject: Re: [Haskell-cafe] Is there a null statement that does nothing?
To: "michael rice" <[email protected]>
Cc: [email protected]
Date: Wednesday, October 21, 2009, 8:49 PM
Yes, an if statement must have both 'then' and 'else' branches. As
an example, what if you had
let a = if b == 2 then True else False
and you were missing an else branch? What would 'a' get assigned to?
The if statement "returns" a value so must have both branches.
However, in a monadic constraint, there are the functions 'when' and
'unless.' They allow conditional evaluation of expressions in a
monadic context. For example,
main = do
line <- getLine
when (line == "hello") putStrLn "Hello back!"
Cheers,
- Tim
On Wed, Oct 21, 2009 at 7:43 PM, michael rice <[email protected]
</mc/[email protected]>> wrote:
It looks like both the THEN and the ELSE in an IF expression
must each have an expression. What's a graceful way to do
nothing in either or both slots, kind of like the Fortran
CONTINUE statement.
--mr
================
[mich...@localhost ~]$ ghci
GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> if (1==1) then else
<interactive>:1:15: parse error on input `else'
Prelude> if (1==1) then True else
<interactive>:1:24: parse error (possibly incorrect indentation)
Prelude> if (1==1) then True else False
True
Prelude>
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