Implement infix notation

This commit also involves rewriting many parsers and refactoring the
end-to-end tests.

Author: mkantor

README.md

@@ -164,6 +164,32 @@ and `@runtime`.
 Currently only `@function`, `@lookup`, `@index`, and `@apply` have syntax
 sugars.
 
+#### Infix notation
+
+Infix notation can be used to apply binary functions (like `a => b => …`). For
+example, the expression `x f y` desugars to `:f(y)(x)`. Here's another example:
+
+```
+{
+  cons: b => a => { :a, :b }
+  list: 1 cons (2 cons 3) // evaluates to `{ 1, { 2, 3 } }`
+}
+```
+
+The standard library contains symbolically-named functions for arithmetic and
+other familiar binary operations. For example, `1 + 2 - 3` is `0`. Also
+included in the standard library are the functions`|>` (pipe) and `>>` (flow):
+
+```
+{
+  // `>>` composes functions from left to right
+  append_bc: :atom.append(b) >> :atom.append(c)
+
+  // `|>` pipes an argument into a function
+  abc: a |> :append_bc
+}
+```
+
 ### Semantics
 
 Please is a functional programming language. Currently all functions are pure,
@@ -197,7 +223,7 @@ possible. For example, this program compiles to the literal value `2` (no
 computation will occur at runtime):
 
 ```
-:integer.add(1)(1)
+1 + 1
 ```
 
 There's currently no module system and all Please programs are single files, but

examples/fibonacci.plz

@@ -1,24 +1,23 @@
 {
   fibonacci: n => {
-    @if, :integer.less_than(2)(:n)
+    @if
+    condition: :n < 2
     then: :n
-    else: :integer.add(
-      :fibonacci(:integer.subtract(2)(:n))
-    )(
-      :fibonacci(:integer.subtract(1)(:n))
-    )
+    else: :fibonacci(:n - 1) + :fibonacci(:n - 2)
   }
 
-  input: { @runtime, context => :context.arguments.lookup(input) }
+  input: {
+    @runtime
+    context => :context.arguments.lookup(input)
+  }
 
-  output: :apply(:input)(
-    :match({
-      none: _ => "missing input argument"
-      some: input => {
-        @if, :natural_number.is(:input)
-        then: :fibonacci(:input)
-        else: "input must be a natural number"
-      }
-    })
-  )
+  output: :input match {
+    none: _ => "missing input argument"
+    some: input => {
+      @if
+      condition: :natural_number.is(:input)
+      then: :fibonacci(:input)
+      else: "input must be a natural number"
+    }
+  }
 }.output

examples/kitchen-sink.plz

@@ -4,7 +4,7 @@
   bar: :foo
   sky_is_blue: :boolean.not(false)
   colors: { red, green, blue }
-  two: :integer.add(1)(1)
+  two: 1 + 1
   add_one: :integer.add(1)
   three: :add_one(:two)
   function: x => { value: :x }

examples/lookup-environment-variable.plz

@@ -1,17 +1,15 @@
-{@runtime, context =>
-  :flow(
-    :match({
+/**
+  * Given CLI arguments like `--variable=FOO`, looks up the environment
+  * variable named `FOO`.
+  */
+{
+  @runtime
+  context =>
+    :context.arguments.lookup(variable) match {
       none: {}
-      some: :flow(
-        :match({
-          none: {}
-          some: :identity
-        })
-      )(
-        :context.environment.lookup
-      )
-    })
-  )(
-    :context.arguments.lookup
-  )(variable)
+      some: :context.environment.lookup >> :match({
+        none: {}
+        some: :identity
+      })
+    }
 }