[Haskell-cafe] Writing guards shorthand

Jules Bean jules at jellybean.co.uk
Thu Apr 19 09:06:18 EDT 2007

Joel Reymont wrote:
> Support I want to infer the type given an Op that looks like this 
> (incomplete):
> data Op
>     = Minus
>     | Plus
>     | Mul
>     | LT
>     | GT
> Is there a shorthand way of bunching Minus, Plus and Mul in a function 
> guard since they all result in TyNum whereas the rest in TyBool?

data NumOrBool = JNum | JBool deriving (Eq)

numorbool Minus = JNum
numorbool Plus  = JNum
numorbool Mul   = JNum
numorbool LT    = JBool
numorbool GT    = JBool

f o | numorbool o == JNum = TyNum (...)
    | numorbool o == JBool = TyBool (...)

...so you have to define one function rather verbosely (numorbool) but 
once you've done it once you can use it in a guard to define others more 

You could use pattern guards instead of == and forget about deriving Eq 
if you prefer.


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