haskell - Defining function signature in GHCi -


defining function signature in haskell's interpreter ghci doesn't work. copying example this page:

prelude> square :: int -> int  <interactive>:60:1: error:     • no instance (show (int -> int)) arising use of ‘print’         (maybe haven't applied function enough arguments?)     • in stmt of interactive ghci command: print prelude> square x = x * x

how can declare function signature , give function definition in haskell interactively? also: why can't evaluate function , see type (e.g. prelude> square) once has been defined?

you can define function signature in ghc interactive shell. problem need define functions in single command.

you can use semicolon (;) split between 2 parts:

prelude> square :: int -> int; square x = x * x

note same holds function multiple clauses. if write:

prelude> is_empty [] = true prelude> is_empty (_:_) = false

you have overwritten previous is_empty function second statement. if query empty list, get:

prelude> is_empty [] *** exception: <interactive>:4:1-22: non-exhaustive patterns in function is_empty

so ghci took last definition single clause function definition.

again have write like:

prelude> is_empty[] = true; is_empty (_:_) = false

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