[clean-list] Passing an environment around

[Pieter Koopman]

Hello Romildo and clean-list

At 05:12 18/10/00 -0200, Jos=E9 Romildo Malaquias wrote:
>Hello.
>
>I am implementing a Computer Algebra system (CALG) in Clean, and I have a
>problem I would like the opinion of Clean programmers.

...[snip]...

You can use the curried functions to implement infix operator to implement=
=20
infix operators with 3 (or more) arguments.
An example from the standard environment is:

(o) infixr  9
(o) f g :=3D=3D \ x -> f (g x)

A more elaborated example is a very simple evaluator for expressions=20
constisting of values and names:

------------------
module calg

import StdEnv

:: Memory :=3D=3D [(Name,Value)]

read :: Name Memory -> Value
read name [] =3D abort ("No value for "+++toString name)
read name [(n,v):r]
         | n=3D=3Dname
                 =3D v
                 =3D read name r

:: Operator :=3D=3D Memory -> Value

//name :: Name -> Operator
name :=3D=3D read

value :: Value -> Operator
value c =3D \mem -> c

(.+.) infixl 6 :: !Operator !Operator -> Operator
(.+.) e1 e2 =3D \mem -> e1 mem + e2 mem

(.*.) infixl 7 :: !Operator !Operator -> Operator
(.*.) e1 e2 =3D \mem ->  e1 mem * e2 mem

:: Name :=3D=3D String
:: Value        :=3D=3D Int

f :: Operator Operator -> Operator
f x y =3D x .+. y .+. y

Start =3D (value 2 .*. f (value 7) (value 1 .+. name "y" .*. name "z"))=
 amemory
where
         amemory =3D [("x",1),("y",2),("z",3)]
---------------------

If you want to prevent the double evaluation of the second argument of the=
=20
function f you can replace the function by:

fopt :: Operator Operator -> Operator
fopt x y =3D \mem -> let vx =3D x mem; vy =3D y mem in vx + vy + vy

Does this help?

Have fun, Pieter Koopman