It just means to bind a variable: let x : u32 = 0; In Haskell, it's used for temporary definitions, or pure definitions in do blocks: let f = \x -> x

It just means to bind a variable: let x : u32 = 0; In Haskell, it's used for temporary definitions, or pure definitions in do blocks: let f = \x -> x + 1 in f 2