Elm: How can I find out the type of an Elm expression or a subexpression in elm-repl?

Question

How can I find out the type of an Elm expression or a subexpression in elm-repl ?

Haskell's :type or :t equivalent in Elm REPL?

Answer 1

The Elm REPL automatically prints the type of whatever you enter. For example:

> "foo"
"foo" : String
> f = \a b c -> (a + 1, b ++ "!", c || False)
<function> : number -> String -> Bool -> ( number, String, Bool )
> f
<function> : number -> String -> Bool -> ( number, String, Bool )
> f2 a b c = (a + 1, b ++ "!", c || False)
<function> : number -> String -> Bool -> ( number, String, Bool )

As @amalloy points out, without an equivalent to GHCi's :type command, Elm REPL (as of 0.18) forces evaluation of an expression prior to showing you the type, which may be undesirable for expensive function calls. In its current version, there is no way around this.