16 lines
709 B
HTML
16 lines
709 B
HTML
<h2 id="haskell-types">Haskell Types</h2>
|
|
<p>Sometimes, the type determine a lot about the function<sup>★</sup>:</p>
|
|
<pre class="haskell"><code>fst :: (a,b) -> a -- Only one choice
|
|
snd :: (a,b) -> b -- Only one choice
|
|
f :: a -> [a] -- Many choices
|
|
-- Possibilities: f x=[], or [x], or [x,x] or [x,...,x]
|
|
|
|
? :: [a] -> [a] -- Many choices
|
|
-- can only rearrange: duplicate/remove/reorder elements
|
|
-- for example: the type of addOne isn't [a] -> [a]
|
|
addOne l = map <span class="red">(+1)</span> l
|
|
-- The (+1) force 'a' to be a Num.</code></pre>
|
|
|
|
<p>
|
|
<p><span class="small base01">★:<a href="http://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf">Theorems for free!, Philip Wadler, 1989</a></span></p>
|