Lambda 表示法 function 签名不能在 ghc 中编译，但在解释器 (ghci) 中可以

Question

Lambda notation function signature does not compile in ghc, but OK in interpreter (ghci) I am using stack on mac osx. ghc version is 8.8.3

\x y -> 2*x + y :: Num a => a -> a -> a 
valNum1 = \x y -> 2*x + y 

Answer 1

The problem is that you define a signature for the lambda expression, but this lambda expression is an expression , not a declaration. If you want to specify the signature, then you specify this as the signature of valNum1 :

valNum1 
valNum1 = \x y -> 2*x + y

It is however more convenient to specify the variables in the head of valNum1 :

valNum1 :: Num a => a -> a -> a
valNum1  = 2*x + y

or you can write this as a point-free variant:

valNum1 :: Num a => a -> a -> a
valNum1 =