Why not? -notation (bang-notation) works in the Idris REPL?

I was expecting this to evaluate to 3, but it gets an error instead:

Idris> :let x = Just 2
Idris> 1 + !x
(input):1:3-4:When checking an application of function Prelude.Interfaces.+:
        Type mismatch between
                Integer (Type of (_bindApp0))
        and
                Maybe b (Expected type)

      

I also tried this without toplevel binding and got

Idris> let y = Just 2 in !y + 1
Maybe b is not a numeric type

      

+3


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1 answer


The problem is how !

-notation desugars.

When you write 1 + !x

, it basically means x >>= \x' => 1 + x'

. And this expression does not print the check.

Idris> :let x = Just 2
Idris> x >>= \x' => 1 + x'
(input):1:16-17:When checking an application of function Prelude.Interfaces.+:
        Type mismatch between
                Integer (Type of x')
        and
                Maybe b (Expected type)

      

But this works great:

Idris> x >>= \x' => pure (1 + x')
Just 3 : Maybe Integer

      



So, you have to add pure

in order for everything to work:

Idris> pure (1 + !x)
Just 3 : Maybe Integer

      

Nothing special about Idris's line, this is how type checking works. That's why in the textbook Idris has pure

in m_add

:

m_add : Maybe Int -> Maybe Int -> Maybe Int
m_add x y = pure (!x + !y)

      

+4


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