Expression evaluation tree in Haskell -


in exam today asked create expression evaluation tree in haskell. answer simple as:

data expr = value integer           | add expr expr           | sub expr expr           | mul expr expr

and evaluate it, use function such as:

eval :: expr -> integer eval (value x) = x eval (add l r) = eval l + eval r eval (sub l r) = eval l - eval r eval (mul l r) = eval l * eval r

however today, given data type:

data op = add         | sub         | mul

so assumed create expression tree do:

data expr = value integer           | op expr expr

and use same eval function. however, have since written function , loaded ghci, not seem work. explain why doesn't work?

you must define data constructor (providing name)

data expr = value integer | compute op expr expr                             ^^^^^^^

then

eval :: expr -> integer eval (value x) = x eval (compute add l r) = eval l  + eval r

and on.

:)


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