Question

UPDATE – the context of this question was pre-TypeScript 1.4. Since that version, my first guess has been supported by the language. See the update to the answer.

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I can declare f to be a function that accepts a string and returns a string:

var f : (string) => string

And I can declare g to be an array of string:

var g : string[]

How can I declare h to be an array of “function that accepts a string and returns a string”?

My first guess:

var h : ((string) => string)[]

That seems to be a syntax error. If I take away the extra parentheses then it’s a function from string to array of string.

Answer 1

I figured it out. The problem is that the => for a function type literal is itself merely syntactic sugar and doesn’t want to compose with [].

As the spec says:

A function type literal of the form

( ParamList ) => ReturnType

is exactly equivalent to the object type literal

{ ( ParamList ) : ReturnType }

So what I want is:

var h : { (s: string): string; }[]

Complete example:

var f : (string) => string

f = x => '(' + x + ')';

var h : { (s: string): string; }[]

h = [];

h.push(f);

Update:

Judging from this changeset parentheses will be allowed in type declarations in 1.4, so the “first guess” in the question will also be correct:

var h: ((string) => string)[]

Further Update It is in 1.4!